You've had the hurt, now here's the comfort! You're welcome :D
Edited: Because uploading from mobile broke my formatting. BAH! Hope y'all had a good christmas!

Gaster gives it half an hour before he decides to go find Sans. He tells himself that Sans is probably just taking his time after six days of not being up and about, but he knows him better than that. With how excited he was, he would have done everything in the shortest possible time and ran down here as fast as he could with how excited he'd seemed. He leaves the test room, and backtracks towards the bedroom. On finding it empty, however, a small panic stirs in his chest. Where could he be? He wouldn't have got lost, he knows this place like the back of his hand.

A text interrupts his thoughts before they can snowball. He nearly ignores it, but he's glad that he doesn't in the end.

The quiet mechanical hum in the room fills his skull, but today it's not doing anything to quieten his thoughts. Even with his back pressed against the generator and folded into the tiny corner as he was, his safe spot wasn't offering any reprieve for him today. He hears the door open quietly, and footsteps enter the room. Assuming it's just someone checking up on the Genny, he pulls his feet up and tucks himself completely out of view. Whoever it is gives a puzzled sigh, before the sound of bone on plastic. Oh joy, it's dad. The person he wants to see least while he's feeling like this. He waits with his breath held so as to keep himself completely still. The text tone chirps into the room.
"Sans, where are you?" He counts to sixty in his head, but Gaster doesn't move. He slowly lets his feet drop back down to the floor from the wall, and pokes his head around the piece of machinery.
"'m here."
Gaster takes a seat so he's got his back against the generator too, facing the room inwards. "You'll have to remember this place next time you and your brother play hide and seek."
"nah, the idea of hide and seek is hiding somewhere you can be found."
"Hmm… pretty sure the opposite is true."
"ha, not when you're playing with pap. i hid too good once and it took me an hour to calm him down when he thought that he'd lost me forever."
Gaster can imagine exactly that, and has to laugh at the thought of it. "How long had you let him look?"
"six hours. we'd had an argument the day before and the idea seemed hilarious at the time. until the tears started, that is."
Gaster laughs harder. "Where did you hide that he couldn't find you for six hours?"
"here and there." He hears the light smirk in his voice.
"You cheated?"
"nothin' in the rules against moving hiding spots. you've just gotta stay not found."
"I'm pretty sure that it's one of the biggest unspoken rules of hide and go seek that you stay in one place."

"nah, it's like a video game. the rules that limit the game can be changed, if you know how to without breaking it." Gaster chuckles as he shakes his head. "'sides, i'm pretty sure texting the guys watching the cameras is actually cheating."
Gaster replies without missing a beat "Well, I didn't know we were playing. You can change spots if you like."
"...nah, you'd never find me again. it's only pap that i let win." He grins, feeling a little better.
"Maybe some other time then."

They sit in silence for a minute or two, before Gaster decides to ask.

"You seemed pretty excited to show me what you've learned, so how come you decided to come here instead?"
"i like it here."
"That didn't really answer the question."
"i know." Sans sighs quietly; so quietly that Gaster almost missed under the whir of the machinery that powers the lab until the CORE is beyond theories and research.
"If you're not feeling up to it, there's always another day."
"i … i'm not still tired." Gaster waits patiently, but the thought isn't expanded upon. He gives his own sigh.
"When did we stop talking, Sans?" He asks, feeling a mixture of sadness and perplexed. He gets a confused noise in his direction. "It wasn't that long ago that you would tell me anything, given enough time and space for you to know what you wanted to say. Now it feels like you know exactly what you want to say, all the time, but there's never enough time and space for you to feel comfortable enough to do so."
"so? what do you want me to say?"
"I want you to talk to me. To let me help you instead of secluding yourself until you think the problem's gone away."
"it's nothing."
"And yet here we are."
"...okay then, it's nothing you can help with." He rephrases after a moment.
"You can't know that without letting me to at least try and help-"
"maybe I don't want your help. did you think about that?" The words are quiet, but they hit Gaster like a truck. He runs a hand over his face, pushing away the snap of frustration threatening to bubble to the surface. And doing that instead of engaging the harsh words like he might normally, makes him realise that this is exactly the kind of situation that Monster Guy had been talking to him about.
"That's quite enough. I know you don't mean that and you know you don't mean that. It's incredibly frustrating that you'd rather build walls and shut me out with this stoic bullshit instead of letting me try and help. I'm not leaving until you tell me what's troubling you."

He gets no response, except the quiet rustling of fabric as Sans pulls his feet under him so they nearly touch his hip joints, wrapping his arms around his legs and resting his chin on his knees. He doesn't know how to respond to that. There's a shifting sound, but he doesn't turn to see what it is.

"...You're going to get me yelled at again." Gaster tells him, a smile quirking the corners of his mouth, and Sans chuckles quietly.
"pap?"
"No, Guy has been riding my tail."
"dan dan the dino man?" And despite himself, that makes Gaster laugh heartily.
"You really should be nicer to him. He cares a lot."
He watches as guilt creeps into the eye lights. "i… i know."
"You two used to be friends."
Nothing.
"Does that mean I'm wrong?"
Still no response.

Gaster reaches out, lifting and turning Sans' head to look at him. Somehow he looks young and scared and tired and old all the same time. He wonders how long his son's had that look, and how he's missed it before now. "It feels like I don't know who my own son is anymore, and that… it scares me. You're growing up right before my eyes but I seem to be missing everything important that's happening."
That makes the pupils focus on him, and he stares as if trying to find any hint of a lie. There's none to find. Slowly, Sans turns his body so he's facing his father and Gaster lets go of his face.
"...i scare me too. i think."

That takes a minute to compute, but it seems MG had been right about the honesty for honesty thing.

"That wasn't what I said."
"it's what you meant, though."
"No." Gaster's voice is firm.
"but… why? everybody-"
"-Is wrong. And it's not everybody. Me, Papyrus, and Monster Guy - to name a few - aren't scared of you. For you, maybe, but that's part of caring about someone. And it's a big difference."
He doesn't get an answer, save for looking deep in thought.
"Why are you scared?" he coaxes after a few minutes.
"...don't wanna talk about it." the pupils look away again, staring hard at the wall.
"That's fine." Gaster decides to back off of it for a minute, and try to go around the emotional wall that just slammed up on him. "Then I believe I should take this opportunity to say something else."
The pupils draw back to him.
"I'm sorry."
"for what?"
"Many things. But... if you want specifics, how about we start with the last session in the test room."
"there's nothing to apologize for." Gaster catches the hand that tries to wave it away.
"No, there is. I obviously hurt your feelings, as much as you try your best to conceal them." he lowers Sans' hand into his lap and holds onto it gently. He feels fingers loop through the hole there as Sans places his other hand on top. It's been a long time since Sans had done so, a gesture Gaster didn't realise he's missed from the little one - even now he's not so little. "I was frustrated that you were pushing yourself too hard and putting yourself at risk, but I never should have called your work pointless. I mean, you can dodge like no monster I've ever seen. And whatever you managed to do with Guy had him singing your praises."
He sees the slump leave his posture a little bit, and his pupils brighten a little bit. "really?"

...Did his son really not know? "Yes. I'm so, so proud of you. I'm starting to think I don't say it enough."

A flush rises to his son's cheekbones, the fingers tighten around his hand but not enough to be painful. "but… i… i..."
"Hmm, it seems I definitely don't say it enough." Gaster says out loud, regret layering the words.
"...but I'm not the one that deserves it."

How many more times can Sans surprise him. And break his heart - metaphorically speaking. Suddenly he realises just how much of a pedestal Sans has put his brother on, how much he actually adores Papyrus as the 'I'm' in the sentence clicks with him. He knew that he was Sans' first and only concern, but he'd always thought that he showered 'the great Papyrus' with the praise he did mostly out of some sort of unending patience with playing along. Or to make up for the times that Gaster forgets to say it. Or both.

How had he missed this?

"You both deserve it, and so much more. What on earth would make you think you don't deserve just as much as him?"
"he's just so…" he gets that fond look in his pupils, the gleam that's reserved for his brother. "...so great! he's so cool and awesome, his puzzles are… well, he never stops trying his hardest which is what matters and they're getting better all the time, he never seems to stop enjoying life and nothing can knock him down for long. and he always sees the good in people. always. he's going to grow up to be the best, nicest monster in the underground and everyone's gonna love him as much as I do, i just know it. and i just… just… wish I could… be more like him."
Gaster puts his free hand on top of Sans' and gives it a small squeeze, ensuring that he has the young skeleton's attention. "Sans. Just because you and your brother are different doesn't make you any less deserving of anything. He has his strengths and you have yours." Sans tries to remove his hands, but Gaster holds them where they are, gentle but unmoving.

And finally, Gaster's found his way around that damn emotional wall that his son threw up. It saddens him that it takes longer every time to both see it and work around it, and he worries that if the trend continues what that means for him as an adult.

"like what? being able to see how bad people are, how much hurt they've caused? dodging or running away because one hit's gonna kill me?" He manages to remove his hands from Gaster's grip, and he grips his own wrist so tight Gaster's half worried he might shatter it accidentally. His pupils have completely disappeared. "destruction? none of these are good things. there's nothing to be proud of."
Gaster stares, utterly dumbfounded. "...Do you honestly not see the good in yourself? Do you want me to tell you what I see, as it seems you cannot?"
His son doesn't give any sign of even having heard him, seeming to have fallen into himself and his dark thoughts. He's reminded of when he listened to Sans and Papyrus under a ceiling of fake stars.

"You're going to make the stars cry."

That snaps Sans' attention to him. "you were listening."
"Is that why you're scared?" The shudder through his bones is all the answer Gaster needs. He wraps Sans in magic, and pulls him into him for physical comfort. He rests his skull on top of Sans' as he holds him close. "I'm sorry I didn't see how much you were hurting from it still. But it doesn't define who you are. We all have our moments when things are overwhelming, or we get angry or scared and do things we don't mean to. You have so much potential, and in that you have so much potential for good. You're loyal to your own detriment, once you deem someone worthy of that loyalty. The same can be said for the depths of your kindness. The stats you see don't just show you people's bad but their potential for good. You have more patience than I have ever seen in a child of your age, and your capacity to learn may even outstrip my own if and when you put your mind to it. I don't think I have ever seen you make a promise you don't think you can keep, and if and when you do make one you do everything you can to keep it and bear the consequences. You're older than your years, too old maybe, but it's given you unbelievable wisdom already. And as for dodging and running, there's no shame in that - knowing when to retreat is as important as knowing when to fight. You also move with a speed and fluidity in battle that not many can achieve. That's a whole lot of things to be proud of, my son."

He feels Sans breath a quiet sigh against his chest, but he doesn't look down to try and read into it.

"And when you find yourself thinking about it, you need to remember: you did all that, but nobody got even that. You never hurt me -even as you threw me about- or Papyrus. That's what you should think about."
"Da- mg said that i shouldn't fear myself or my magic, but the minute that i went to the room it was all i could think about. I thought because I fought him, i would be okay with you as well but… but..."
"You have to forgive yourself, Sans. You're the only one that hasn't."

Both Gaster and Sans are out of words, neither being great with how they feel and talking about such things, so neither bothers any more and instead both quietly just hold on to each other. How long had it been since they had even done this? As a child he'd never been as tactile as Papyrus, but when he'd sought out physical affection or reassurance it had meant just as much. At some point along the line he'd stopped seeking it so much, usually obliging his brother's need for hugs and affection but only taking it for himself very rarely and even rarer from his father. It was an absence that Gaster hadn't realised he had missed quite so much as it turns out he did.

He'd just been making mistakes right across the board, huh.

They stay like that for an indeterminate amount of time, in silence save for the gentle hum of the machinery. It doesn't grate on Gaster's nerves like he thought it might and he starts to understand why his son chose to come here. The constant source of gentle noise fills the silence easily in a way that fills his skull unobtrusively - a little like if you turn a radio on between stations (or, just about anywhere in the underground) and listen to the white noise on a low volume. At one point he begins to idly wonder if Sans has fallen asleep but he didn't dare move, taking as much comfort from this as the young skeleton was. He was wrong, with Sans eventually untangling himself and standing up.
"you comin'?" He offers a hand to his father, who takes it. His joints complain at being sat on the hard floor for so long, but he ignores them as they pop. Sans leads the way quietly, to back outside the test room. Gaster reaches out for his hand, gently catching it for a moment. He receives a questioning look.
"Are you sure you're ready?" A firm nod. Gaster searches him for any sign of fear or worry hiding in the pupils and the cheekbones that ever so slightly change the curves of his smile when you know what to look for, but finds nothing. He releases the hand, and they step inside and take their places on the opposite sides of the room.

Sans stretches himself a little, no longer feeling the cold dread that usually flooded him when he stood opposite his father in the room.

i've got this.
don't be afraid.
listen to instinct, listen to yourself.

It's all a part of him and maybe… maybe that was something he can accept after all. Learning to like it would be a slower process, he's sure, but acceptance for better and worse, was manageable.

Besides, even if he doesn't know how to ask for it a lot of the time, turns out he's got help.

we've got this.

Gaster sees the pupils drop out and the flaring of cyan and yellow is the only sign he gets that the encounter is starting. The opening barrage is much more brutal than he thought, having to pull more reflexes out of the bag than he'd been expecting. He's slammed to the floor with a flick of the hand, and he has to push against the slam of gravity quickly as bone rises from the floor to meet him, he's let go mid-jump and the sudden shift in gravity difference means his landing is fumbled, throwing off his footing as he has no time to waste as bone comes at him in waves. It's got a wide path but the fumbled footing means he narrowly avoided taking the attack rather than it passing by him easily, and as he finishes weaving through the bones, blasters are waiting for him at the end already powering up meaning that he has a split-second decision on where to go, and the width of the blasts correct him when he chooses wrong.

"Holy shit, Sans!" He grins, and Sans looks awfully pleased with himself. That and… he's looking like he just used a shitton more magical energy in one go than he used to, as much as the pride and excitement are hiding a lot of it. Did he just… open with the strongest attack combination he could think of? That's ingenuity right there, and Gaster adds that to the list of things he's just so fucking proud of his son for right now. "Not bad at all."

He decides to just *CHECK, seeing if the surface glance reveals any changes now that he's manifested an attack.

*Sans
*HP 1/1 ATK 1 DEF 1
*He's got this.

The comment takes Gaster by surprise, but he's glad of the more positive shift in attitude to the last ones. But he looks at the gap in his HP bar compared to the insight he just gained. He's missing 12 but he got hit once…
Oh. So maybe not poison after all, but-
His HP ticks down a single number, but he sees no pink. A single bone clatters to the floor having just bounced off of his skull. Sans pulls his posture into a teasing mimicry of his father's, pupils back in place and dancing with humour. "can you please focus, dad?" Gaster laughs, brief but hard.
Cheeky little-
He throws out a semi-simple pattern, one that he knows Sans could dodge in his sleep. And sure enough, he's beside him with little effort, brow bone raised over one eye. "c'mon, dan tried harder than that and I hadn't just had a six day nap. dodging I can do."
Gaster would roll his eyes if he had them, and if he had time to. The telltale blue glow of bone is covering the floor and he's got but a moment to react, diving for a small patch not covered and mid leap he's slammed upwards towards another patch of blue on the ceiling. The sudden shift in gravity is enough to throw his landing off and he doesn't push away from the surface fast enough to avoid the magical piercing sensation. Sure enough, his HP bar only takes 1 physical damage, but a chunk of it turns pink and takes a second tick roughly half a second later. He pushes himself away from it and the pink stops racking up on his HP bar, and he's back on the floor. His thoughts are split between the results of his *CHECK and the sudden feeling of the world on his shoulders - not an unfamiliar sensation, but one that's usually reserved for sleepless nights not the middle of a fight. He does his best to detach himself mentally from the edging of his darker thoughts, and decides to step it up. He responds in kind to Sans' attack by wrapping him in blue and throwing him to the side, ready to pull back if necessary, but he sees Sans' adjust himself to work with the momentum that he finds himself travelling with and absorb the impact in a half-roll, using the attack's momentum to continue to where he finds his feet to push against the force of the gravity magic pushing against him before bone and blaster can make contact. Sans' barely finished the manoeuvre before Gaster finds himself in the middle of a cross of blaster maws, springing to the side of the formation and barely clearing the space where the lasers fill he pushes off the ground with the momentum he gains from his dodge and jumps into a sideways platform thrown his way as the entire floor turns to bone of varying size and moves towards him. He spots and leaps to second platform before the one he's on hits the wall, rolling off it and landing in the clear floor after it. He spares half a glance for his HP bar and sees that the pink chunk has taken a fifth of it by itself, despite not taking another hit. Hmm.

His kid's just stood across from him with a shit-eating grin edged with pride, hands in his shorts pockets as he hadn't picked up his hoodie like he hadn't just thrown out four rounds of 'what the fuck'. Gaster holds his hands up, pride in full force now the dark edges to his thoughts having faded somewhat. They're still making themselves known, but are right now being overridden.

"Okay, okay."
"quitting before you have a heart attack, old man?" He grins eye lights dancing.
"Skeletons don't have hearts." He smirks back, then adds "...but I must admit that it's been a long time since these 'old' bones have had to move that fast."He remembers Monster Guy's advice which, so far, had turned out pretty well.

"You did good, kiddo. You did real good."