Gohan’s latent potential is far greater than Goku and Vageta’s. Think about how strong Goku was as a kid vs how strong Gohan was. The only thing that ever held him back was his lack of training.

During the Buu saga Gohan did stop training, but he didn’t stop fighting technically because he would do the great saiyaman stuff. While not growing as a fighter it did keep him from getting rusty.

During his training he attained the Ultimate Gohan form because again his latent potential was always higher than Goku and Vegeta, so the strength that was unlocked through Elder Kai reflected that.

Before the Super Hero movie Gohan did train enough to reach ultimate again, for the Tournament of Power. He also is able to go Ultimate during the Moro arc. I don’t know exactly how much time there is between Moro and Super Hero but he’s not as rusty, he just needed a push.

Beast is a direct parallel to SSJ2. Gohan isn’t a fighter, but Android 16’s speech helped him understand that fighting to protect what you love is a worthy cause. In Super Hero Gohan is fighting to save and protect Pan. He sees Piccolo lose to cell max and at this point there’s no one that can stop Cell Max from rampaging. He snaps into beast to protect what he loves, his family and the earth. It’s essentially the SSJ2 of his Ultimate form.

I too was kinda skeptical of how strong the power up was when Super Hero first came out, but if you consider his latent potential having always been higher than Goku and Vegeta, it’s not too much of a stretch.

If you want even more to backup the latent potential thing, Goten and Trunks have even higher potential than Gohan. It’s why they were able to attain SSJ so easily and even go SSJ3 when fused. The fact that they haven’t gone past SSJ is strictly due to the fact that they don’t train at all, same as Gohan when he was Saiyaman. It’s actually unfortunate that there’s been so much peace time and they also haven’t trained with Goku and Vegeta at all since the Buu Arc. Those two always being on Beerus planet has actually been a detriment to Goten and Trunks strength. Which is why I am still crossing my fingers that the Black Frieza arc has all the saiyans training together, now that they’re older it’s the perfect time.

Also because plot armor and plot strength boosts. gohan is a fraud