Contrary to popular belief, a gun turret is a weapon system with shielding or protection, not simply a mounted weapon system.
The HMMWV to the left, though it has a mounted .50 caliber machine gun, does not have a gun turret, as no protection is afforded for either the user or the weapon's mechanisms. On the right however the weapon is surrounded by protective shielding, making it a gun turret. Many mounted guns in video games do not have this protection, and as a result are not technically gun turrets.
Turrets play a very extensive role in many videogames. Some serve as a perimeter guard against wandering players, a basic enemy, while others serve the purpose to be used by the player.