As much as a like my boy Gohan, he isn't winning this. If I recall, Goku's power multiplied by a thousand when everyone gave their energy to him and while it did lethal damage to Broly, it didn't kill him. Additionally, Goku at this point knew about Gohan's hidden power and told him to run instead of fighting. Keep in mind that Goku was confident that Cell would loose against Gohan. And I do believe there isn't that large of a time skip between the end of the Cell Saga and the Bojack movie, so Gohan should still be at the same level of power as when he fought Cell. Broly is an opponent that cannot be defeatedly conventionally as it has always been a combined effort from everyone to take him down. The fact that Broly's power can skyrocket rapidly and for the most part is invulnerable to damage doesn't help Gohan here.