The Gelth were a race of spectral entities that appeared in the Doctor Who episode "The Unquiet Dead". Originally, these beings were corporeal and had a humanoid appearance, but their physical bodies were destroyed when their race became caught up in the crossfire of the Last Great Time War. Though their physical forms were lost, the Gelth survived as gaseous beings and endured this state until they could find a new environment to thrive in.
During the Last Great Time War, billions of Gelth were turned into gas. Eventually, they discovered the Rift, which opened into Gabriel Sneed's mortuary in Cardiff, Wales on Earth in 1869. They were attracted to the area, since they could inhabit the gas found in the pipes and emerge through gas lamps. They also discovered that they could survive in the gases in a decomposing human body and, as the Ninth Doctor put it, "use it as a vehicle". The Gelth contacted Gwyneth, Sneed's servant, who had psychic ability. She thought of the Gelth as angels. Through Gwyneth, the Gelth spoke with the Doctor.
Presenting themselves as peaceful and facing extinction, the Gelth asked for the Doctor's help in allowing Gwyneth to open the time rift further, allowing through more of their kind. The suggestion of doing this and allowing more Gelth through the rift to inhabit the corpses of humans was an idea which horrified the Doctor's companion, Rose Tyler. The Doctor, however, accepted the idea as an interim stage before taking the Gelth to a place where he could help them acquire new, specially-created bodies. With the rift opened wider, the Gelth, however, planned to let all the billions of Gelth through and take the planet by force. Gwyneth closed the rift in time, however, and destroyed the remaining Gelth with fire.
Though the Gelth originally had biological bodies, they were eventually reduced to a gaseous form during the Time War, after their bodies were destroyed during the conflict. They could shape this gas into a wraith-like form, but they needed to be in a gaseous environment to survive. Once there, the Gelth emitted ghostly screams as they floated around a place they congregated at.
The Gelth could take control of human corpses, though only with difficulty. While inhabiting the corpses, the Gelth could control and move the dead humans as walking zombies, leaking glowing blue gas that streamed through the mouth. While controlling a human, they tended to continue unfinished business. The Gelth would eventually be forced out of the corpse over time or if there was a lot of gas in the air around them.