I think that "gunners" are more prevalent in schools where specializing is a popular path. The combination of that with a ranking system often leads to some pretty cutthroat competition (so I've heard). I've asked this question at every dental school I interviewed at, and it seemed like it existed to some extent at every school, but that students often learn quickly that this is not an effective method for success. It sounded like at every school, there are more people who are willing to help you than not, although there are the few exceptions at EVERY school unfortunately.