Despite all his machismo, Gladio is actually a needy bottom, and the real reason he leaves the party in Chapter 7 is to get his hole wrecked. He's ashamed of it interfering with his duty as the King's Shield, but he's at his limit and fingers/toys just won't cut it. Cue a consensual gangbang with a crowd of strangers (how he finds or gathers them is up to you) manhandling and using him, one after the other until he's a begging, gaping mess.

Bonus
+ Someone like Cor or Dave showing up for the fun and being surprised to see Gladio
+++ But not too surprised to give it to him good