In the manga version of Dragon Ball Super , Future Trunks is a student of his timeline’s version of Shin. Thanks to Shin’s guidance, Trunks is able to actively control his ability, using it to not only heal himself but also keep others alive. This keeps his allies like Future Mai safe and healthy whenever they are faced with danger.

As for Gohan, he trained with Kai for a short time in his own timeline. Therefore, unlike Future Trunks, Gohan does not have the ability to heal, because Kai did not teach him this. This may be a special ability that Shin privately taught Trunks when he discovered his power potential.

However, the fact that Gohan only trained for a short time under Kai’s guidance is not the entire reason why Gohan cannot use his recovery ability. Maybe Gohan himself has the same potential as Future Trunks, but because he doesn’t spend much time practicing, that ability doesn’t develop. Just like we saw how Trunks spent a lot of time and effort during his training with Shin.

It can be seen that Gohan and Future Trunks are both people with strong potential because they are both a combination of Saiyan and human blood. However, it will only be maximized when they practice diligently and have a reasonable instructor. Therefore this is the reason why Future Trunks can use regeneration and Gohan cannot.