A person (usually a medical or law school student) who uses over 3 different colored highlighters, tabs every page in their notebook, and raises their hand after every question asked by their professor, regardless of if they know the correct answer or not. Gunners like to hear themselves speak. They use complicated words to make themselves sound smart even though they have no idea what's going on in class- they pretend they do. They are trying to intimidate you and eliminate competiton.
This gunner in my Chemistry class answers every question the professor asks, asked 5 questions during a 1hr class, and uses 8 different colored highlighters.