The Gas Mask is a piece of equipment featured in Grand Theft Auto V.
The gas mask is used to protect the protagonists from suffocating on any airborne gases. The gas mask is used prominently in the Jewel Store Job (if the smart approach is taken) to keep Michael and the gunmen from inhaling any of the knockout gas.
Unfortunately, the player cannot use the gas mask outside of this mission.
It is also utilized by Bugstars Pest Control.
The full face gas mask is based on the real life MCU-2/P.