Gunners are, more often than not, people you wouldn't like much anyway, even outside of the med school environment. So if you see him at, say, a bar, you might think "what a tool."
If you see him in class asking some b.s. question about some inane detail of something that the entire class understands only as a big picture and will obviously only be tested as a big picture, and he's clearly asking not because he gives a care in the world about the answer, but because he wants to show the whole class how much more he knows than them, you might think "what a gunner."
(And you still might think "what a tool.")