Gas masks were devices that allowed humans to resist the effects of poison gas when worn.
During the Thousand Year War, both the Kaleds and the Thals wore gas masks when engaging in combat. When Harry Sullivan came across a dead combatant who was wearing one, he remarked that it seemed out of place next to the radiation detector he was also wearing. (TV: Genesis of the Daleks)
In 1941, a plague caused by the Chula nanogenes made anyone infected grow a gas mask for a face, thinking it was an organic component of the human body. (TV: The Empty Child/The Doctor Dances)
In 2008, the Master, disguised as Harold Saxon, sat safely wearing a gas mask when he set a toxic gas in the Cabinet Room and killed all the members of the Cabinet. (TV: The Sound of Drums)
In 2009, gas masks were used by UNIT during the Sontaran attack on Earth, as the Sontarans' ATMOS devices were releasing clone feed gas. (TV: The Poison Sky)