The gas mask protects the Protagonist against the harmful Dust gas that plagues the streets of Haventon. Gas masks can be found throughout Haventon on lifeless bodies. They help the Protagonist with his scavenges throughout the devastated city by causing his Stamina bar to drop slower.
The Protagonist picks up the gas mask upon leaving Henry's Apartment the second time.
Although the gas mask allows the player remain in the dust environments for longer, the stamina bar will continue to drop until it begins to effect the players health.