Gohan is pretty darn strong in the manga. He has moments of extreme power, especially when he unlocks his hidden potential. However, his strength isn't always consistent and depends on various plot developments and training.
The relationship could develop out of a need for companionship. Gohan, having lost many of his friends and family in battles, is in need of someone who can understand his pain. 18, being an android with her own complex past, also longs for someone to be close to. They meet and initially just offer each other a listening ear. As they talk more, they find that they can comfort each other in ways no one else can. This emotional connection then grows into a romantic relationship, and finally, they marry as they realize they don't want to be without each other.
Gohan may become more confident in relationships. In a harem situation, he has to deal with multiple people interested in him, which could boost his self - assurance.
In the long run, they might realize that they need to change their behavior. They could start thinking about how to make it up to Chichi. Maybe they'll go and do some nice things for her, like getting her some flowers or cleaning the house really well without being asked. This would show that they've learned their lesson and are trying to be better sons.
In a Gohan x Goku fanfic, their relationship can be delved into by creating scenarios where they have to rely on each other completely. For example, if they are in a situation where Goku's strength and Gohan's intelligence are both required to escape. Their different personalities can also be highlighted. Goku's carefree nature contrasted with Gohan's more studious and responsible side. And of course, flashbacks to Gohan's childhood and how Goku's presence or absence affected him can add depth to their relationship.
Gohan's powers would be quite unique in the Marvel Universe. His Super Saiyan forms would give him a massive boost in power, far beyond what most normal Marvel characters can achieve without some sort of external boost. His Zenkai boosts, which increase his power after near - death experiences, could also be an interesting concept to introduce. And his martial arts skills, combined with his Saiyan instincts, would make him a very adaptable fighter against the various threats in the Marvel world. He could also potentially teach other heroes his fighting techniques.
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.
He might be a bit flustered at first. Gohan is not the most confident when it comes to romance. He could stutter a bit and then take a moment to compose himself before responding positively. He might say something like 'I've always hoped for this, but I'm just so surprised right now' and then go on to express his love for her.