The Suppressor, or silencer, is a device attached to the muzzle of a firearm to deafen its firing sound and suppress its muzzle flash; it also limits the recoil of the gun. It is used on some weapons in the GTA series but comes standard on them and cannot be removed. However, in GTA V, the suppressor can be used as an optional attachment for most weapons.
The silencer first appeared in Grand Theft Auto 2 on the Silenced Machine Gun. The silencer's only purpose is to make pedestrains, including cops, oblivous to to the gunfire.
In both Grand Theft Auto: Vice City and Liberty City Stories, the suppressor appears on the PSG-1, though it serves no purpose other than athstetics.
In Grand Theft Auto: San Andreas the silencer appeared on the Silenced 9mm. The silencer serves the purpose of stealth, as firing the silenced 9mm will not alarm any pedestrians, unless the pistol is pointed directly at them, nor will it attain you a wanted level, unless fired very near a police officer.
In Grand Theft Auto: Vice City Stories the silencer appears on the Mac 10, though it serves no purpose other than to make pedestrians and cops oblivious to gunfire.
The suppressor makes appearance again in Grand Theft Auto: The Ballad of Gay Tony but this time on the Assault SMG. The suppressor serves no purpose other than to supress the muzzle flash of the weapon.
The suppressor is an attachment in Grand Theft Auto V that can be mounted on most weapons: