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. Similarly, she later saw much of her relative future through the mind of Rose Tyler, and even knew that her father was dead, even though she was never told that. These psychic powers enabled her to be contacted by the gaseous Gelth who had arrived in the city via a space-time rift located there.
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. (TV: The Unquiet Dead)
Gwyneth's genetic traits were passed, through the space-time rift via spatial genetic multiplicity, to her distant relative Gwen Cooper, an agent of the branch of the Torchwood Institute based in Cardiff in the 21st century. Gwen and Gwyneth looked practically identical, although Gwen did not inherit her psychic abilities. (TV: Journey's End)
Gwyneth was a kind and caring individual, quickly becoming friends with Rose Tyler shortly after having met her. However, she was also very reserved, discreet and shy, and would quickly change the subject when someone would begin questioning her about her abilities. Her naivety also unfortunately allowed the Gelth to easily manipulate her into serving as a gateway for them. In the end, she displayed great courage, sacrificing her life to stop the threat of the Gelth. (TV: The Unquiet Dead) The Twelfth Doctor noted that she was a "brave lady". (PROSE: A History of Humankind)
Gwyneth was a telepath who could read people’s minds. She knew how the Doctor liked his tea and saw what 21st century London would look like after reading Rose’s mind. However, she appeared to have little, if any control over this ability and often read people’s minds unintentionally.
She was also clairvoyant and could see and sense elements that normal humans could not detect. She sensed the impending arrival of the Ninth Doctor and could detect the alien presence of the Gelth in Mister Sneed’s home as well as the Cardiff Rift. (TV: The Unquiet Dead)