A gas mask is a mask worn over the face to protect the wearer from inhaling airborne pollutants and toxic materials. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Some gas masks are also respirators, though the word gas mask is often used to refer to military equipment (e.g. Field Protective Mask, etc.)
Claire Redfield had to wear the Gas Mask to enter the Drill Room, which was flooded with Toxic Gas at the time.
Notably, USS agent HUNK wears one at all times.