He knows it's a lie, but they don't have a choice in the matter. Prompto might catch the faint look of 'proud of you' in Gladio's eyes or not, but it's there, as he shifts enough to try and make sure that Prompto does not in fact headbutt him, then takes his hand to hold it before pouring on the potion. This time, given the somewhat dramatic reaction of last time, he braces himself more by tucking his shoulder right behind Prompto's head and uses the side of his head to keep the blond's there while keeping a grip on Prompto's hand. At least he can protect himself from some flying body parts, this way.

He's damn proud of how Prompto's handling all of this. He could be broken, angry, scared, and no matter what he is, he's fighting to keep going. Noct needs Prompto, hell they all do, but Gladio knows that like Ignis did, Prompto's got to make that choice to stick with them. Otherwise, the fight won't be in him to keep going through this, but... how they're going to make it to the end, he has no idea. Find the crystal, that's the only path forward he knows right now. Noct has a destiny and for better or worse, they're there to make sure it gets seen through to the end.

Right now, though? Once the pain wears off for Prompto, he might notice that Gladio has let go of his hand and unthinkingly is stroking the skin along his forearm with his thumb, trying to be soothing.