During Combat taking place in a building (such as during Break-Ins, Prison Breaks and Escapes) the use of Gas Grenades can flood a room with knock-out Gas. Enemies walking through a gas-filled room will be stunned as long as the gas remains in effect - but the gas will also stun Max, potentially rendering her unconscious (and subsequently Captured by the enemy).
This can basically render some rooms impossible to enter for a considerable period of time (up to a minute or more, depending on the selected speed). While the gas can create a barrier, keeping enemies away from Max, it denies her free movement through a room, and thus has limited use.
A Gas Mask renders the above point moot: While worn, it allows Max to safely ignore any effect from Gas whatsoever! You can even stand inside a Gassed room for the entire duration of the Gas without any problem.
Gear SelectionEditLike any other piece of Gear, a Gas Mask can be brought along for a Break-In by selecting it in the Gear Selection menu.
It takes up 1 Gear Slot, out of the total 5, like any other piece of equipment. If Max is already carrying 5 pieces of Gear but not the Gas Mask, the mask can only be selected if at least one piece of Gear is de-selected first.
Once the Break-In starts, it is possible to acquire a Gas Mask by collecting one from a Floor Safe. There is no guarantee that any Floor Safe will yield a Gas Mask, nor is there any guarantee that any Floor Safe will even exist in the Hideout you broke into.
As long as it is in Max's inventory, the Gas Mask is always in effect. This means that you can freely walk into Gassed rooms without problem for the duration of the current Combat mission.
Tactically, there are many uses for a Gas Mask. You can render a room impassable to enemy guards, while retaining the ability to move into and out of that room freely. This means you can even ambush enemies in that room, as they become stunned shortly after walking into it - while you yourself do not.
Note that while it is possible to collect Gas Grenades from Neutralized Enemies, enemy guards never throw them at all. Only Max can lob Gas Grenades. However, it is not required to have a Gas Mask to do this - just make sure you do not walk into rooms you've Gassed, unless the Gas has already dissipated.
Losing the MaskEdit
If Max is knocked out and Captured during the course of a Break-In, and decides not to Exchange Agents with her captors, she will be spawned in the Agent Room in the same building, without her Gas Mask (nor any other item other than the mandatory Pistol and Kevlar Vest).
However, you may still add a Gas Mask to your Gear in future Break-Ins, as normal.