In the Games you will encounter quite a few varieties of gas.
A plain example of gas is in the Derelict Apartment Block of Tomb Raider: The Angel of Darkness, where Policemen throw gas canisters to spread gas around the building. Lara can breathe small amounts of this gas without taking any damage. When her air bar has run down, she will take the full damage. In the Louvre level Galleries under Siege, you can get a gas mask, from a cabinet in a room down the hallway by Margot Carvier's office. It will make reaching the end of the level much easier for you, although you can do it without the masks.