Canonically, future Gohan's biggest foes were the androids that weren't even a Cell level threat and from the saga we've seen that Saiyan strength grew with how strong the foes were (god level was reached after beerus, a god, came into the picture. Ultra instinct was a result of a being stronger than gods etc). Present Gohan was fortunate enough to be trained by Goku to master the super saiyan form which came in handy when ascending to super saiyan 2. Then came bigger threats like Buu that ultimately resulted in Gohan having his full potential unlocked by the old kai, giving him a big boost. Stronger foes came, opportunities to train and push past limits came in form of tournaments against strong opponents. And let's not forget...the writer's choice to make present Gohan stronger maybe to usher in the new main protagonists of the DB saga alongside Broly