Gladio is in love with Ignis, but as hard as he tries Ignis continues to see him as just a friend. That changes when Noctis disappears and Ignis goes to him for comfort and they enter a relationship. Gladio is overjoyed at first but one night Ignis mutters Noctis's name in his sleep or something. Gladio realizes Ignis has been in love with Noctis all this time and still is, and he feels like he's being used a a rebound. Lots of drama ensues.