Ever since he came back to Kame-House after his little 'escapade' with Tora by Gingertown, Gohan was forced into isolation by Chichi, so he could focus on studying. She even didn't let him tell Krillin, Yamcha and Master Roshi about what was going on when he and Tora left after receiving that strange photo.
That same afternoon he took out the sketchbook he had lent to the time traveler and spent a moment looking at the scribbles linking the two separate worlds together. The whole thing was far more interesting than the labels he needed to write on his fauna workbook. He turned the pages back to the drawing he did almost three years prior of the Time Machine, retracing the word 'Hope' he'd copied without understanding it then; he'd since learnt the meaning of the word, and it just carried so much more than he could fathom at the time. He then used another page to draw the cicada moult he'd seen that day.
Later that afternoon, he overheard the phone call asking everyone to meet up at the Lookout, and before the evening came, he felt a giant clash of energies that worried him sick. It was short lived, however, and he recognized the energies as being like his father's. Tora and Vegeta, by elimination process, as Goku was right there, and Gohan himself couldn't reach super Saiyan. He overheard Yamcha and Krillin discussing that same clash outside, with Master Roshi. The turtle hermit reassured his two students that it would be ok as he felt no malicious intent from either fighter. Coming from Vegeta, this surprised Gohan, but he trusted the older and more experienced adults more than his own worries. An hour later, he sensed Krillin and Yamcha leaving the island, and he waited impatiently for their return. When they returned, far later during the night, after his mother had fallen asleep next to his father, he opened the window of their temporary room and sneaked outside to listen to the conversation.
When Yamcha began explaining the fact there's another Time Machine, and that this was that Time Machine's third trip, out of which came out something but nobody knew what it was, and Trunks was 'only informing them to keep an eye out for threats', Roshi walked to the side of the house to fetch the spying Gohan, and just brought him along in the conversation. He looked mortified for being caught red handed eavesdropping, but Krillin and Yamcha just welcomed him. Krillin had promised he'd fill the kid in, after all, and they reassured him they wouldn't tell Chichi. Gohan confirmed whatever Tora said on the Lookout, including the dates he'd read out loud on the console earlier, and apologized for not being allowed to tell them when he returned because, well, homework.
Since no adult mentioned Tora's state of mind when he told them about the whole ordeal, Gohan guessed the teenager hadn't opened up about his doubts with them like he'd done with him, so he decided to respect that and kept it a secret. Gohan didn't really understand the 'wider implications' as the teenager had explained while smoking and looking at the moult. Sins, existing twice and 'shouldn't be here' was just too much to fully grasp, yet. Gohan only felt the other was lonely, misunderstood, and that he had done 'sinful acts' by changing history. Gohan couldn't accept that, Tora was a hero who saved his father's life, who killed Freeza, and who treated him like his own person, and not 'Son Goku's son' or 'just a kid'. He felt a connection the moment Tora had told him there was nothing shameful about missing his father, almost three years prior. Even if he had only spent a couple of weeks in the teenager's company then, Gohan found himself trying to sit closer, get his praise, see his eyes, be there for him.
His heart pounded hard in his chest when he saw him again that same morning, he wanted to follow him to find Vegeta and Dr Gero, letting Bulma and Yajirobe fly back home on their own with the car, but Tora had specifically told him to go with them, so he obeyed, trusting they would succeed. Sadly they hadn't, but the senzu beans saved their hide.
In the plane to change Goku's location, Gohan decided to do the homework he was tasked with near Tora, and the teenager accepted his presence as if it were natural, as if it was meant to be, and he found the smell he'd enjoyed prior but somewhat more pronounced. Probably due to Tora being straight out of combat, and he hadn't smoked. There was something about the time traveler's natural scent, after a few days without tobacco on the Son estate, that Gohan couldn't quite put his finger on, and right there he could feel so much more of it. When the teenager borrowed the sketchbook, Gohan slid under his arm to see and just found peace with their kis so close. He admired Piccolo and his father, and wanted their praise, approval, their attention, and some of their personal space and touch (Gohan loved hugging his father, which is what had made his initial contact with Tora so embarrassing), but somehow, Tora treating him as equal, asking to 'fight together', made it feel different. His mind went absolutely blank when Tora pulled him into a hug when Gohan assured he'd fight for him after he'd expressed his worries and fears when confronted to the strange cicada moult. He happily hugged back, heart full, and decided he would figure out exactly what his feelings meant when he was older. At the moment, at almost nine, he just wanted to help and protect this lonely hero, so close to his age compared to everyone else, and whose distress shattered his heart. No amount of stress-smoking would keep him away, even if he resented the tobacco from veiling the smell he'd grown into. Thankfully (and because Tora was mindful when smoking near a child), Chichi hadn't smelled the cigarette on Gohan when he arrived back at Kame-House. It'd been a long day .
When discussing the new Time Machine with Krillin, Yamcha and Master Roshi, eventually Gohan asked about Piccolo's whereabouts. Only Tora, Krillin and Yamcha came back to move Goku, so everyone else he had no idea about. Krillin reassured they were all fine, and that Piccolo was staying at the Lookout. Gohan didn't ask why, and Krillin didn't tell him. It seemed the Mafuba was the technique chosen by the fighters to deal with the Androids once they'd found them again, but so far, they would just avoid them as long as they caused no harm. Master Roshi seemed to approve of the idea, but he wasn't sure whether they'd work on artificial beings like the Androids. Gohan corrected him, Krillin and Yamcha about them being Cyborgs, not Androids (like Nineteen and perhaps Twenty were). Gohan relayed the 'augmented humans' comment Tora had made, but to Yamcha, this was a potato/potato comparison, and the result that as Cyborgs, their ki couldn't be sensed, meant that perhaps the Mafuba wouldn't work on them. Master Roshi doubted it would, but he agreed to prepare the seals nonetheless.
The following few days were frustrating. Gohan liked reading and studying, but his worries were elsewhere, and waiting suddenly profoundly bothered him, almost as much as seeing his father suffer and his mother worry. The medicine was helping, clearly, and he was grateful it was just a temporary problem. He'd soldier through it, he'd be the man while his father was incapacitated, and he'd ease his mother's worries. And if that meant pretending to study, then so be it. The child kept tabs on the kis he could feel from that remote location. On the bright side, Goku seemed much more peaceful, sometimes he even smiled, and he managed to wake long enough to wolf down a meal. The second call from Capsule Corp gave them an update on the information found on the Time Machine's diagnostics, and as Krillin relayed it, Gohan knew Tora had kept quiet some of the more complex elements. The time traveler only confirmed that the Machine came from another universe, and filtered all other information, guarding his secrets and worries to himself.
The day before his birthday, which he didn't care to celebrate considering the circumstances, in the evening, Gohan felt yet another super Saiyan clash. Yamcha, Krillin and Roshi also felt it, and the four of them stood in anticipation. Yamcha stopped Gohan from rushing to the door. The child tried to argue, this clash felt so different, desperate even, and something primal wanted Gohan to go. He only calmed down when Vegeta's ki extinguished. From experience, Roshi could guess the Saiyan had been knocked out.
On the day of his birthday, in the morning, when Bulma called him to congratulate him on his ninth, Gohan asked about what had happened the day prior. He could feel Bulma was reluctant to tell him, so he said he felt the fight. Bulma eventually told him Vegeta had finally come back to Capsule Corp and straight up forced Trunks into a fight in the Gravity Room, which was cut short when Bulma herself shut down the system. She told Gohan Trunks was 'a tough cookie' and would be fine if he took it easy for a few days; she trusted her teenager's knowledge of his own healing process.
Bulma called later that same day to ask the gang to turn on the TV and watch the news. What they saw horrified them.
"-town of Gingertown. We sent a team to investigate and found that all inhabitants have mysteriously disappeared! There was no trace of anyone. At this time, the cause remains unknown… As you can see, clothes litter the streets. Wait, what's this?" screams, and static.
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Gohan's birthday is on May 18th :) I'm taking Toriyama's early mistake in the chapter where he introduces himself as being 3 when he first meets Raditz and the gang (pushing his birth year from 757 to 758); this was later corrected but I like Gohan juuuust a little younger.
So Gohan turns 9 during the Android arc, and is almost 10 during the Cell Games (due to the one (almost) year in the HTC).