Gladio leaves all sorts of very conspicuous hickeys on Noctis's neck during one of their make out sessions or during sex.

Can be pre-game or during the Brotrip,

If pre-game:
+Regis sees and demands how Noctis got those hickeys, and a blushing Noctis has to lie his way out
+Gladio treats Noctis to something special because he just endured hooorrible embarrassment

If during the brotrip:
+Ignis and Prompto notice. Prompto is like ":O whaat" but Ignis nags onto Noctis because he's getting married, what are you doing Noctis??
+Gladio confesses their relationship to the bros