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 will most likely encounter other Z - fighters like Krillin and Tien. They were all part of the group back then. Also, there's a good chance he'll meet Master Roshi. Roshi was a key figure in Goku's training, so his interaction with Gohan could be really interesting. He might also come across Bulma, who was always involved in the Dragon Ball story, either by accident or because he needs her help with some time - related technology or something.
One possibility is that it could be a genetic anomaly. Maybe something in his Saiyan genes that was dormant suddenly activated, causing the tail to grow back. In the Dragon Ball universe, Saiyan genetics are complex, and there could be some unexplored factors at play.
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.
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.