Gelth is a race of gaseous alien, which came to Cardiff in 1869. They were possesing dead bodies, which had a lot of gas inside, the perfect host. The Gelths had a plan to kill all the humans and live in a world full of gas. The Doctor met them, and stopped them to invades the world through a crack in the Necroterium.
Gelths can phase solid objects, like ghosts. They can possess dead bodies, and walk through gas. They can enter in fire and have superstrengh while possesing a dead body.
Gelths only can walk free through gas. When a lot of gas is disspared they have to leave their hosts. They can be blew-up.