Gohan himself could be a main character in some form. Maybe he exists in Naruto's memories or as a spirit guide. Another potential main character could be Sakura. She might be interested in Naruto's new powers and try to help him understand and control them. And of course, there could be new characters introduced from Gohan's world that interact with the Naruto characters.
In a 'naruto son of gohan fanfiction', perhaps Naruto is born with a special power that is a combination of his own ninja abilities and some latent power from being related to Gohan. This could lead to him being an outcast at first in the ninja world as his powers are so different. But as he grows, he learns to master them and use them to protect his village and the world from new threats.
It could be a new Saiyan from another part of the universe who is jealous of the power the family holds. He might come to Earth to challenge the son and prove his own superiority.
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.
I'm not aware of any such fanfiction. The relationship between Grandpa Gohan and Son Goku in the original story is one of love and mentorship, so this kind of dark and negative portrayal in fanfiction is not common that I know of.
In this fanfiction, Gohan as a girl could bring a new dynamic to her relationships. She might have a different kind of friendship with Videl. Instead of the typical male - female relationship they had before, it could be more of a sisterly bond where they support each other in different ways, like in fighting crime together in a more united front.
This idea is quite out - there in fanfiction. One way it could work is if there are some powerful deities or beings in both universes who decide to manipulate the fates of characters. They could use their powers to transplant Naruto's essence into a new world where Gohan and Videl exist and make them his parents. However, this would completely change Naruto's origin story and his character development would have to be re - imagined in the context of the Dragon Ball world, which has very different power systems and cultures compared to Naruto's world.
Perhaps Gohan's own overconfidence could lead to his death. He might underestimate an enemy's power or a dangerous situation and make a wrong move that results in his end. For example, he could rush into a fight without proper preparation thinking he can handle it easily.
Naruto would probably be the most interested. He loves making new friends and Gohan's power would surely catch his attention. He'd be eager to learn about Gohan's world and his abilities.