Until The Fire Eternal, Gwendolen does not seem to show any special ability. We find out that she is a IT dragon and can connect her self to computers with her tail. She can do this because she is a mirror of Lucy and Lucy loves computers. Also in The Fire Eternal, Gwendolen steals a letter that Zanna had written to David after his 'death', and shows it to Lucy. Gwillan spots her and tells the other dragons, who force her to tell of her crimes.
In The Fire Within, David Rain, the main protagonist of the series says that when he looked deeply into Gwendolen, he could see Lucy. This is very symbollic of the connection between Lucy and the Dragons, as we know that the human Gwendolen was the first daughter of Guinevere and Gawain, and Gwendolen is one of Lucy and Elizabeth Pennykettle's ancestors.
~ Gwendolen is proud to be called "the best 'GPS' in the world, even though she doesn't know the term.