In Canada and the United States, a "gunner" is slang for a hyper-competitive student, usually a medical, nursing, legal, or dental. Gunners will do whatever it takes to be the best. This ranges from working very hard to sabotaging peers. The gunners are normally either concerned with getting into a professional school or excelling in the professional school to continue on to a specialty.
The top student in your class, anyone sitting in the front row, anyone who uses a three colored pen in university/college.
Gunner: *Gives correct answer no one else knew*
Average college student: "Of course he/she knew the answer, he/she is such a 'Gunner'!"