Well, Gohan's powers would change the power dynamics a great deal. His ability to fly would give him an edge in battles and mobility. His Super Saiyan forms would be extremely intimidating. For example, if he were to go Super Saiyan in a fight against a devil or an angel, it would shock everyone. His powers might also attract the attention of the more powerful beings in the High School DxD universe, who would either want to recruit him or eliminate him as a threat.
Gohan's super strength would make him a force to be reckoned with. He could easily overpower most of the regular characters in High School DxD. His ki blasts could also be a new form of attack that the others in the universe have never seen before.
Gohan's powers from Dragon Ball would have a huge impact on the High School DxD universe in fanfiction. His martial arts skills, combined with his superhuman strength, speed, and ki - based abilities, would make him a unique and powerful figure. He could use his Instant Transmission to quickly move around the school or in battles, which would be a very unexpected and difficult - to - counter ability for the High School DxD characters. Also, his power level could potentially disrupt the existing balance of power between the different factions, as he would be a wildcard not bound by the rules and politics of the devils, angels, and fallen angels.