The gas mask is used to circumvent the effects of the decontamination gas used in the lockrooms or the gas catwalks. The gas will normally cause the player to cough and make them blink faster, but using the gas mask will help ignore these drawbacks, but will obscure the player's vision when doing so.
When put through SCP-914 on "Fine" or "Very Fine", the Super Gas Mask is made. When it is equipped, it will constantly replenish the player's stamina, allowing for long periods of sprinting while equipped. It will counteract the negative effects that the ballistic vest has on sprinting.
If the gas mask is combined with an ingot of SCP-148 and put through SCP-914 on "Fine" or "Very Fine", it will produce the Heavy Gas Mask. This variant of the gas mask grants the player immunity to the effects of SCP-012, SCP-895, and SCP-966; but the sprinting boost is gone.