Wait until med school. Gunners in undergrad are more like tadpoles than frogs -- they haven't achieved the full level of ugliness. A gunner is NOT someone who works hard and does well. They are folks who try to get ahead by pulling others down around them. They will be the ones who jump in with things you don't know to show you up on rounds or during your PBL presentations. They may volunteer you for undesirable tasks because " I already did one of those". The tools gunners use to make folks look worse then them are unlimited, and most obvious during the clinical years. In law school we had gunners who would tear out the pages of library reserve texts after doing a project so nobody else after them could complete it. You don't want to be a gunner. Now the terms gunner and slacker are tossed around in fun, much like commie and fascist used to be, not so much for the definitional reason but for ribbing purposes, and that's why there is so much confusion. But the short answer is that if you aren't taking overt actions to screw over others you aren't a gunner.
I like this one. 
