The Gunslinger is a fast skirmish bot that's designed for efficiently destroying most T1 units and some lighter T2. It's equipped with repulsive shells, meaning every shot can push lighter units around or stagger larger ones. Its self-repair ability coupled with a decent HP pool makes it self-sufficient in battle.
Gunslingers require good micro to be worthwhile, especially to avoid getting swarmed.
Gunslingers' key feature is the unique way experience affects them. Where most units gain only extra max health and firing speed, Gunslingers also gain longer weapon range, helping them pick off units more safely.