>>111614050"Mind the gap" is a somewhat famous expression used primarily in London subways to warn passengers to keep a safe distance from incoming trains.
In this case, Dobson is jokingly referring to the age gap between younger moviegoers excited to see superheroes fight and older moviegoers confused about why they're fighting at all.
The weird part of this strip is that Dobson isn't really making a point about anything, he's pointing out something and just kind of leaving it at that. He seems to be criticizing Hollywood, but I'm not exactly sure for what. Not appealing to old people, I guess?