I don't think Gladio would have anything against Noct and Iris being together if they really liked each other. They are ridiculing that scary big brother trope themselves in the game.
Still unknown and not too heavy indication that's he's alright with it. However he implied medium sarcasm and inept joke when Noctis picked a picture of Iris, imply case that he's alright with it now since times have changed.
If it takes a will to marry someone for the sake of the country, Gladio will jump start that in a pinch if it ties to the king. If there wasn't such a event, he'd probably be ok with it.
If it takes a will to marry someone for the sake of the country, Gladio will jump start that in a pinch if it ties to the king. If there wasn't such a event, he'd probably be ok with it.