See, I think the term has different definitions to different people. I am the definition of a gunner to some and not one to others.
For instance, I ask questions, but only because I genuinely want the answers. I do the extra credit papers, but I do it so I don't have to kill myself on the final. I often set the curve, but I don't study a ridiculous amount, I just test well.
I care about my grades, and I'm very involved, and people who don't know me think that I must spend my entire life locked up in a study room (which I VERY rarely do at all).
BUT, I actually have a very active life, and while I want to do the best I'm capable of, not at other's expense. I often lead free tutoring sessions for students who ask, and share my notes and homework help with practically the entire class. I also generally do not share my grades unless people ask.
SO, I seem to be a gunner academically, but I lack the personal goal to destroy and demolish all others... Your call.