After Vegeta trains in the Hyperbolic Time Chamber, he comes out not as strong as he would if he trained with his son. He’s still strong enough though to handle Semi-Perfect Cell, and still lets him absorb Android 18. Vegeta fights and loses to Perfect Cell. After Krillin revives him with a senzu bean, Vegeta feeling stronger after the Zenkai boost fights the perfect android again, yet still loses, this time his life.
Scouring the Earth, Cell looks for Goku to give him a good fight but can’t sense him while he’s training in the Hyperbolic Time Chamber. He even hijacks a studio and goes on TV demanding that he will exterminate all life on Earth if Goku doesn’t show himself. By the time Goku and Gohan got out of the time chamber, Chi-Chi, and all their friends including Piccolo after he reunited with Kami were all killed by Perfect Cell. This of course meant that there was no Dragon Balls to bring anyone back to life.
Goku was enraged and actually goes SSJ 2 before Gohan. He fights Cell and the two appear to be evenly matched. That’s until Cell unleashes his Cell Juniors who overwhelm Goku. As Goku falls to his knees and Cell is about to deliver the finishing blow, Gohan’s fury erupts, and he transforms into a SSJ 2. He dismantles the Cell Juniors and brings Cell to his knees with a thunderous kick to the gut. This causes Cell to spit up 18. Cell tries to self-detonate, Goku who can’t get to his feet and instant transmission Cell off the planet tells Gohan to blasts him off the planet with all your power. Gohan powers up for his most powerful Kamehameha and blasts Cell into space where every piece of him is destroyed.
The battle is over, but with no Dragon Balls because of Piccolo’s demise, Goku heals up and uses instant transmission to travel to New Namek. There, he uses the Namekian wishing orbs to wish back everyone who was killed by Cell on Earth. As things finally got back to normal, Trunks in the future timeline never would get stronger if he trained with Vegeta in the time chamber and was killed by Androids 17 & 18. What’s worse is that the Cell of that timeline finds and absorbs the Androids and achieves his perfect form. He destroys the Earth and eventually moves on to other worlds.
Meanwhile in the present timeline, Goku who is alive in this scenario talks with Chi-Chi about letting him train Gohan in case something like this ever happens again. Chi-Chi bargains that Gohan’s studies are the most important thing yet agrees to let Gohan train 30% of the time and study the other 70%. By the time Gohan starts high school, he is far stronger than 7 years ago when he defeated Cell. During the pre-Majin Buu arc, Gohan kills Debura and stops Buu’s resurrection from happening. Heading back to the World Martial Arts Tournament arena, Goku and Vegeta have the best match in the tournament’s history, but the fight ends in a draw due to both warrior’s feet hitting the outside of the ring. Hercule ends up winning the tournament by bribing any Z-Fighter that he faced.
By the time Beerus arrived on Earth, Gohan who had a family and job as a professor slacked off on training and actually became weaker also due to not having his hidden power awakened by the Elder Kai. He could reach SSJ 3 level and do instant transmission like his father, but he was still weaker than if Trunks came back to the past during the Android saga and things happened as they did. Not standing a chance against the God of Destruction, Goku would be the one to become the SSJ God and fight just as it played out in the series.
During the Resurrection F arc, Gohan who felt powerless against Beerus begged Whis to train him, offering a delicious meal in exchange. Whis agreed and trained Goku and Vegeta as well. Gohan didn’t have as much free time to train because of his job and new daughter Pan to help raise. As Frieza came to Earth, Gohan who could now reach SSJ God form fought Frieza while Goku and Vegeta were off training with Whis.
As SSJ God Gohan fights Frieza who was in his final form at 100%, he soon got overwhelmed by Frieza’s massive boost of power in such a short time. Since Gohan hasn’t been training as much as Goku and Vegeta, he hasn’t learned SSJ Blue like them, and is outmatched by Frieza who hasn’t even gone golden yet. That’s until Goku and Vegeta arrive, and things play out as they did once more.
During the Goku Black arc, since Future Trunks is dead, he can’t come back to the past to warn Goku and the others about Goku Black who along with Zamasu invade the current timeline. When the two beings merge using the Potara earrings, they seem all but invincible. But this time, it’s Gohan who learns the Evil Containment Wave from Piccolo and seals Merged Zamasu away for good.
In the Tournament of Power, Gohan who does more training with Whis beforehand can now go SSJ Blue. He lasts to nearly the end but eliminates himself by holding Dysbo so that Frieza can blast them both out of the ring. This part played out the same even though Gohan was stronger, he couldn’t match Dysbo’s incredible speed.
To sum it all up. Gohan would continue to balance work life, family time and training with Whis to stay strong. Vegeta and Bulma would never know that their 8-year-old son is the same person as the one who came in a time machine years ago. Well, until Trunks got to be a teenager and his face and voice changed, then they put two and two together and smiled. As for Goku, he loves to eat a ton and fight strong opponents. No matter what matter timeline were talking about, that will always remain the same.