Weapon Workshop

The Weapon Workshop is a mod shop featured in Grand Theft Auto Online as part of the Gunrunning update.

Grand Theft Auto Online
As the name implies, it is a workshop dedicated to weapons customization, operated by the Weapon Mechanic. It can be found on either the Mobile Operations Center or the Avenger (if the player adds the Weapon Workshop module when purchasing one of the aforementioned vehicles).

The Weapon Workshop basically consist of a medium-sized stand with several tools on the wall, with the prop of a Heavy Sniper in the middle presumably during the customization process.

Grand Theft Auto Online
Selected weapons can be carried to the Weapon Workshop to be upgraded into Mk II variants, which offers an array of customizations not present on their standard models, including ammo types, unique attachments and aesthetic options (livery and colors).

Items from the Gunrunning research intended for the Mk II weapons can be found here, in the corresponding Mk II weapons. Those featured in the Doomsday Heist are already unlocked.