One common theme is adventure. Since Gohan has his Beast form, they might go on adventures together fighting enemies. Another theme could be romance, exploring their relationship deeper.
Well, another place could be some dedicated Dragon Ball fan forums. There are often sections where fans share their own fanfictions or recommend good ones. Some popular Dragon Ball forums might have threads specifically about Videl and Gohan Beast's relationship in fanfiction form. You could also ask around in those forums if you can't find what you're looking for right away.
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.
It could be many characters. Maybe it was Videl. In the Dragon Ball series, Videl is a strong - willed girl, and she has a connection with Gohan. So in the fanfiction, it might be her that Gohan thought was cute.
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.