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The Gelth were a humanoid species who lost their bodies in the Time War.
Biology
Though the Gelth originally had biological bodies, they were converted in a gaseous form during the Time War. They could shape this gas into a wraith-like form. The Gelth also needed to be in a gaseous environment to survive.
The Gelth were capable of taking control of Human corpses, though only with difficulty. While inhabiting the corpses, the Gelth could control and move the dead humans as walking corpses which leaked out luminous blue gas. When controlling a Human, they tend to continue unfinished business. As well, the Gelth would be forced out of the corpse over time or if there was high amounts of gas in the air around them. (DW: The Unquiet Dead)
History
During the Last Great Time War, billions of Gelth were somehow turned into gas. Eventually, they discovered the Rift, which opened up into Gabriel Sneed's mortuary in 1869. They were naturally 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 and as the 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, an idea which horrified the Doctor's companion, Rose, but which the Doctor accepted, as an interim stage before taking the Gelth to a place where he could help them acquire new, specially-created bodies. With the rift actually now open 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. (DW: The Unquiet Dead)
While attempting to bluff the Sycorax into leaving Earth alone, Rose Tyler tried to invoke the authority of the Gelth, among other species she had encountered on her travels. (DW: The Christmas Invasion) Rose Tyler also suspected the Gelth for the Cyberman invasion of London. (DW: Army of Ghosts)