"You have to stop this," said Whis. "I gathered you were the one messing with the timelines and universes since you asked me for information, but the Grand Priest doesn't know yet. I haven't told him of this, I respect you, since you are the son of Goku whose friends make tasty food."
"But Whis, what if we could change the universes for the better? Of everyone? Not like Zamasu, I'm not him." Gohan cautiously said. If anything happened, he could use a backup Gohan from a previous timeline. He'd know he was gone.
"Hmm," Whis pondered. "I will let you do this thing, but I'm neutral. If you want, I can help you train." Whis wondered if Gohan's potential would be enough. Enough to stop even The Grand Priest, if trained.
Gohan trained with Whis. He saw that two factors mattered in fights. One was simple powerlevel, that he trained with Whis. It could be increased by a balance of attention and going with things, Whis said, what Vegeta and Goku had respectively. Two, he noted, was anger. Angry could increase powerlevel, specially for some (it powered up him in the past and Trunks as he fought Zamasu).
Gohan was angry that the world couldn't be good enough. That the laws of the world were not good for all beings. He thought of rationality, he was angry, and his power level increased.
"Hmm." thought Whis. "Perhaps he can defeat and replace The Grand Priest. If not by strength, that he has too, he also has cunning."