Gas masks were devices that allowed humans to resist the effects of poison gas when worn.
History
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)
British soldiers in World War I were given gas masks as part of their standard outfit. (COMIC: The Weeping Angels of Mons)
On 20 January 1941, (TV: Everything Changes) a sign on the doors between a ward and a storeroom in Albion Hospital read "GAS MASKS ARE FRAGILE" in red. (TV: The Doctor Dances)
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. This was later fixed by The Ninth Doctor when he corrected their mistake after reuniting the Child with his mother. (TV: The Empty Child/The Doctor Dances)
The Master and Harry Mailer wore gas masks while they used gas grenades to take over Stangmoor Prison. (TV: The Mind of Evil)
In 2008, the Saxon 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. These gas masks had night vision but could not see through the fog. (TV: The Poison Sky)
Jack Harkness suggested gas masks too late as the 456 ambassador released a deadly virus into the atmosphere of Thames House. (TV: Children of Earth: Day Four)