Gas maks are devices invented in World War I to enable Humans to resist the effects of posion gas. They were also used by civilians in World War II. They were also a prominent feature of The Empty Child and those infected by it. (DW:The Empty Child/The Doctor Dances) Gas masks were also handed out during the Sontaran attack on Earth in 2008. (DW:The Poison Sky).
The Doctor refernces his encounter with The Empty Child by saying "Are you my mummy" when handed a gas mask during the Sontaran attack.