For years, he watched them.
He watched, overjoyed and sorrowed, as Papyrus woke up his happy and energetic self, accepting Sans's explanation of them being orphans without question. He watched them live in the garbage dump in Waterfall for several years before Sans was just old enough to get a job, wanting Papyrus to have a HOME instead of a lean-to.
He watched as Sans used his teleportation ability to work several jobs around the Underground at once until he earned enough to move to Snowdin, where there was a room for rent with a nice Monster couple who were thankfully mostly blind, not wanting to answer more questions than necessary.
He watched as Papyrus flitted around Snowdin like a happy little bird, greeting everyone cheerfully, only to be avoided or ignored, because why on earth would a five-foot-six Monster be wearing stripes like a child? He watched, heartbroken, as Sans urged him to ditch the striped shirts and wear solid colors because 'they don't understand, little bro', hiding his anger from Papyrus, who only wanted to play with the other kids, as any eleven-year-old would want to.
He watched as—at last—a glimmer of good opened up, when Sans walked into town for shopping and ran into his old friend Grillby, who immediately insisted they come over for dinner. The Flame Monster family welcomed the long-lost Skeleton brothers with open arms, sharing their joy at seeing them again, and sorrow when they informed the brothers that Sparks had passed away just months before they came, having been caught in a snowstorm and snuffed out.
He watched as Papyrus became determined to do something worthwhile and cunningly used his height to disguise his age, getting a sentry position at twelve years old. At thirteen, he saw Royal Guards make their rounds and become enamored. At fourteen, he began training his magic. At fifteen he gained complete mastery over his own body and magic. At sixteen, he hit his peak height at seven feet tall, towering over most everyone else in Snowdin, and was still avoided because of his childish demeanor.
He watched as Sans began losing sleep at fifteen, using as much time as possible to fill in his many part-time jobs. At sixteen, he ran into Alphys in Hotlands and all but threatened her to keep quiet about his whereabouts. At seventeen, he realized he had stopped growing. At eighteen, he watched Papyrus training in the forest, using an elegance he used to once see in his father, feeling a pang of jealousy in his soul, his hand brushing the rib under his jacket that had been nicked by a magic arrow when he was nine years old, which had drained and crippled his HP for life, it seemed. At nineteen, he bought the house from the elderly Monster couple, who wanted to retire to Waterfall, and finally, at last, felt that he could rest.
He watched the rest of the Underground too.
Undyne training with all the vigor of her mother before her, climbing ranks in the Royal Guard quickly, and, just as Naiad had prophesied, became Captain of the Royal Guard at age nineteen.
Alphys became Head Royal Scientist, and began snitching human cartoons from the garbage dump to fill a void in her life.
And Asgore...oh...his old friend...
Asgore's son Asriel finding a human child in the old Ruins and helping them home. Asgore's sore heart taking the child in, treating them just like another child of his own. The child growing sick...the child dying...
The horror of watching Asriel take his human sibling's soul and becoming a demigod, taking the body up above, and coming back in tatters before turning to dust.
He watched as Asgore's heart broke to pieces, his anguish driving him to declare any human that fell to be taken to him for their souls to be harvested.
He regretted ever having asked Asgore if he could take the life of a child to save everyone. He regretted even more watching Asgore gain the support of the Underground, but lose his wife, having no one to turn to for comfort when he took the life of that first child.
All he could do was sit there, hands hovering over the king's prostrate form, wishing that his will to comfort Asgore was enough.
It wasn't.
He had to watch again, and again, and again, as the same song-and-dance replayed five more times, with five different young lives. It never got easier for Asgore, EXP gained or not. His old friend was just too tenderhearted by nature to let LOVE take over his life.
Sometimes, selfishly, he wished Asgore would let it change him. He didn't deserve this suffering, and he especially didn't deserve it alone.
And then...everything changed.
It was on a day where nothing should have been odd. Perfectly normal, perfectly boring...and then the air changed. It changed in the way that he even felt it shudder through the Void. This was an air of destiny.
It was another child, but immediately, he knew this would not be like the other times. This child was different. This child had something that made the very Void tremble. It made him pay attention and watch.
While he watched, he could feel thousands of different timelines spread out, and by a mere glimpse, he shut the timelines away and refused to look at them again. To see his loved ones...his SONS...be subjected to THAT...!
So he focused on this one. It was...the best one. It held the most promise..
But the child needed help.
He knew Sans would be around the echo flowers, as the younger Skeleton normally was when a stressful time happened, and he urged his son to help the child, to steer them to the RIGHT path.
The efforts paid off in spades, even when he could do no more than watch the horrors that child faced, the horrors that his friends and family were drawn into.
But that child...they had DETERMINATION.
And that DETERMINATION broke the Barrier.
…...he couldn't go with them.
There was nothing up there for him. The Barrier was gone, and his children would be going onto better things.
Sans, the cheeky thing, courting Toriel (F̸͟͠or͡ ́͝͞ģo҉͠͡o͜͠d͏n̢͘é̸̶s̷͡s̵̢ s҉̡a̛͡͏k͢͝e,̡̨͢ ͘͜͝s̡͟o͜͠n̕͡,͡͞͞ ̸́s̴̴͠h̶ę̀'s͠͝ ̴̧o̵͟l͝҉҉de̴̛r̷ ͟͞th̀͟ą̴̸n҉̛ ̸I̷̴͘ ̛͢͡a҉m!̛͏ ) and possibly having a hand in raising the child...Papyrus expanding his horizons and finding new hobbies and interests...it was all he wanted for them. To be happy.
Gaster spent so long strolling through the empty Underground, losing track of time as he wallowed in the solitude. Almost every time, he ended up back at his sons' home, their presence still there even though it had been emptied when they moved to the Surface.
He would give it time. Given time, he would forget about them, and finally give himself over to the Void, where he would finally feel nothing...
All of a sudden, he felt an agonizing throb in his soul, a painful jerk that screamed out in response to the echos that called do it.
It meant that his boys were in pain, and that they needed him!