How do you get a gas mask Call of Duty 4 Modern Warfare?

In the single player campaign the player cannot use a gas mask. It is completely useless what so ever. It is only cosmetic at the least.

In multi player, the player can be seen wearing a gas mask from a third-person perspective if they have a certain primary weapon equipped, such as an MP5 sub-machine gun or a W1200 shotgun.

The player on the SAS team will be wearing a dark gas mask and helmet on their head, whilst on the Spetsnaz team the player can be seen wearing only a gas mask on their head. It is most likely the Russian made GP-5 gas mask.