Gwyneth's parents died of influenza when she was twelve and she was brought up by Mr Sneed. She went to school only once a week and gained £8 a year. She grew up on top of a rift in space-time located beneath Cardiff, and as a result, she had developed a certain degree of clairvoyant abilities. When Gwyneth gave the Ninth Doctor a cup of tea, she said, "Two sugars, sir. Just as you like it", although she had never met him before. These psychic powers enabled her to be contacted by the gaseous Gelth who had arrived in the city via the Rift.
Gwyneth and the Doctor agreed to the Gelth's pleas to let them come through to Earth and possess dead bodies in order to survive. The Gelth lied to them about the extent of their numbers and their hostile intentions. When she stood underneath an archway in order to contact the Gelth, they passed through, and trapped the Doctor and Rose. Learning the Gelth's true intentions, Gwyneth was able to hold them inside the house, though not to send them back. To destroy them, she struck a match once the Doctor and Rose had escaped, igniting the gas in the chamber and destroying herself in the process. The Doctor would have lit the match himself, but realised that Gwyneth had been dead since standing in the archway, despite remaining able to think and speak.