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.
In a crossover with the Avengers, Iron Man could be an ally. His high - tech gadgets combined with Gohan's power would be a great team. Also in the Pokémon world, Ash could be an ally. Ash's knowledge of Pokémon and Gohan's combat skills could be useful. In the Bleach universe, Ichigo could be a strong ally for Gohan, with their combined sword - fighting and energy - blasting abilities.
If Gohan were in the Naruto world, he would be an extremely powerful force. His Saiyan strength and abilities like flight, super strength, and energy blasts would be quite out of the ordinary in the Naruto universe. He could potentially help in battles against the Akatsuki or other major threats. For example, his Kamehameha wave could be as powerful as some of the most destructive jutsu in Naruto. Also, his training with Piccolo and Goku in martial arts would give him an edge in close - combat situations, which are common in the Naruto world.
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.
One possibility could be that in the fanfic, Android 18 sees Gohan's kind heart and strength. She might start to develop feelings for him as she watches him protect the Earth. Their relationship could be an interesting mix of her android nature and his Saiyan - human hybrid nature.