After considerable thought- almost all the time they have- Gongenzaka chooses ascension. It is visibly not an easy choice for him. Ultimately he determines that the ability to protect what he loves- and the ones who follow even- outweighs the consequences. In his words, he's seen too much to be able to choose otherwise.

With Yuuya's choice, it's even more painful- but Gongenzaka and he spent their time talking for that reason. Neither of them wish to leave the other behind- but he's resolved that this will not be that, not in the way most would think. They'll just...have their time while it's there.

(It helps perhaps that once upon a Portland, he thought this would happen to a point anyway. It does not stop Yuuya from crying about how unselfish Gon is next to him, and there is probably a few cyclical arguments about how much better the other is.)