This was where Gladio wished he could use Noct as a go between. But that would indeed be telling. And he gave his word, which was important. So he considers the advice he could give.

"He's got a brother, right?"

Siblings were often a good way in. Gladio knew about Iris's crush on Noctis (though she hadn't know the other way around). No doubt Ignis had known of Noctis's in return.