It's not suitable. Fanfiction should be a form of creative expression that respects the original work and its fans. Gay sex fanfic in the context of 'hunterxhunter' may be offensive to some viewers or readers. It's better to focus on more positive and inclusive themes that are in line with the spirit of the anime, like friendship, adventure and character growth.
Friendship is a wonderful theme. You could write about the deepening of the bonds between the main characters. Like how Gon and Killua's friendship evolves as they face more challenges together in the Hunter world. It could involve them having heart - to - heart conversations, supporting each other in battles and learning more about each other's pasts.
It's a complex story involving moral conflicts and personal growth. The main character has to navigate through various challenges and make difficult choices.