A gun is a sort of weapon that fires projectiles through a barrel.
In the PPC, guns are limited to use only in continua where they are appropriate and exist. This is the main reason many agents use swords and other types of melee weapons, though guns aren't completely unheard of.
Agents that work in continua that involve the military tend to prefer guns. The most common type of gun used by agents seems to be a pistol or some other kind of handgun, but larger firearms have also been shown in use.
Use of guns in inappropriate canons is uncanonical and, if the weapons and/or shell casings are found by canon characters, could potentially contaminate the canon. This is obviously a serious charge if done by a Sue.