Well, maybe Gohan gets angry because Hercule takes all the credit for saving the world when in fact Gohan did a lot of the real work. In fanfiction, the writer might play up this injustice, making Gohan seethe with anger at Hercule's false bravado.
Cell - like characters are often antagonists. Since Cell was such a major foe in the original series, new versions of Cell - like beings could be created in fanfiction to make Gohan angry. They might target his loved ones or the Earth itself, provoking Gohan's rage.
Another scenario might be when someone undermines his power or mocks his Saiyan heritage. Gohan, having grown up with a sense of pride in his Saiyan roots, would get angry and show his true strength to prove them wrong. He might also get angry if he sees innocent people being bullied or oppressed, as he has a strong sense of justice.
One possible reason could be that Videl does something that endangers herself. Gohan, being protective, might get angry out of fear for her safety. For example, if she recklessly jumps into a fight without proper training or preparation.
However, it could also be a turning point. If Gohan realizes that his anger is misdirected or if he communicates his reasons for being angry to Videl, it could strengthen their relationship. They could learn more about each other's boundaries and grow closer as a result.
It could be due to unfair treatment from teachers. For example, if he's blamed for something he didn't do just because he's a Saiyan or has special powers. Teachers might be prejudiced against him, which would lead to his anger. He's a good - hearted person but constantly being misjudged would surely get on his nerves.
Perhaps Hercule tries to train under Gohan in a really clumsy and disrespectful way. He doesn't follow Gohan's instructions and constantly brags about his own non - existent skills. This could lead to Gohan snapping at him out of frustration for his lack of discipline and respect.
Obviously, Hercule could be seen as the antagonist in this case. His actions or words are what lead Gohan to snap. For example, if he's constantly hogging the spotlight that should belong to Gohan and the other real heroes, it would make him the source of conflict.
In some fanfictions, Gohan's anger might fuel a rivalry between them. They could compete in various ways, like in battles or in winning the respect of the public. This rivalry could add a lot of drama to the story and change the dynamics of their relationship completely.
It could be a new, unknown enemy. Maybe someone from a far - off planet who has a grudge against the Saiyans. This character could be really powerful and sees Gohan as an easy target at first, so he throws a punch.