The Gelth appeared in “The Unquiet Dead”, Series 1 Episode 3, where The 9th Doctor and Rose met Charles Dickens and the dead were coming back to life.
The Gelth are gaseous life forms who claimed to be few in number. They explained that they lost their bodies in the Time war and needed bodies to live; they couldn’t sustain their life on Earth for very long. Corpses of dead humans were just laying there with no purpose but the Gelth could use them.
Mrs Peace, an elderly woman was the first to be possessed by the Gelth and went to see Charles Dickens’ play “A Christmas Carol”, Charles Dickens described one of the ghosts that Scrooge encountered as just looking like Mrs Peace, when she went blue, releasing the Gelth from within, to the public. Charles Dickens couldn’t believe what he was seeing. The Gelth are fast and are attracted to Gas such as fire lit lamps.
Gwyneth was needed to open the rift in time and space and let the Gelth through so they could use the bodies, however the Gelth didn’t turn out to be what they said they were. They weren’t few in number there were many in number, and they weren’t good life forms neither they were bad. Using corpses, The Gelth acted like zombies, strangling people and making other Gelth possess them.
The Gelth looked sweet and friendly at the beginning, things to feel sorry for, and The Doctor fell for the trick and let them through the Rift. When evil the Gelth turned a ferocious red from a calming blue. (Can be seen below)
A common phrase of the Gelth’s was “There’s not much time, pity the Gelth.” Pity they got and it didn’t do anyone any good. Gwyneth had to sacrifice herself to close the rift. However the rift didn’t stay shut as seen in Torchwood, Doctor Who “Boom Town” and Doctor Who “Utopia”. So maybe the ghostly Gelth escaped from the rift and are out somewhere, finding bodies and harming other species. The Gelth may still be alive.