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.
In some fanfictions, it could be an evil version of one of Gohan's friends. For example, an evil Piccolo who has been corrupted by some dark magic. This Piccolo - like figure might punch Gohan as part of a plot to take over the world or something similar.
Well, in a 'Gohan gets punched' fanfiction, it might start with Gohan in a tournament. He's facing a really tough competitor. This competitor, using a sneaky move, punches Gohan right in the gut. Gohan feels the pain but uses it as motivation. He then analyzes his opponent's fighting style more carefully. He dodges the next few attacks and counter - punches with his own signature moves. It could be a story about Gohan's resilience and his ability to learn from being punched.
Another possibility is a jealous rival. Maybe there's someone who is jealous of Gohan's power or his relationship with certain characters. This rival could do things to annoy Gohan and push him to the edge of his temper.
Yes, there are quite a few. You can find some on fanfiction websites. Some might focus on Gohan meeting Saitama and the comical or action - packed situations that could arise from their encounter.
Piccolo might also be a savior. He has a soft spot for Gohan since they trained together. He could use his strategic mind and powerful attacks to outwit Raditz and rescue Gohan. Maybe he teams up with Goku for an even more epic rescue mission.
One common theme is power comparison. Writers often explore how Gohan's power from Dragon Ball would stack up against Saitama's in One Punch Man.
Maybe an antagonist who wants to disrupt the group dynamic. If Annabeth is a key part of a group, punching her could cause chaos and confusion among her friends. It could also be someone who has been influenced by dark magic or some sort of mind - control in the fanfiction. This would make them act against their normal nature and punch Annabeth without really having a personal grudge.
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.
He could face Jiren again. But this time, Gohan would be the one to come out on top. Maybe he'd have studied Jiren's fighting style thoroughly and found a way to counter it.
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.
Maybe Chi - chi is constantly comparing Gohan to Goku in the fanfiction. Gohan has his own identity and abilities, and being compared all the time can be frustrating. He could get mad because he wants to be recognized for who he is, not just as Goku's son. Also, if Chi - chi is interfering too much in his training or friendships, that could also lead to his anger.