In the short story Sanctuary, Gladys is a former maid of Miss Marple's. She has a husband or boyfriend named Ernie. He tells her that everything good she has learned, she has learned from Miss Marple and so encourages her to help Miss Marple whenever she can.
In the story, Gladys helps Miss Marple and "Bunch" to set up a trap for criminal elements who were searching for a bag belonging to Walter St John. Following Miss Marple's instructions, Gladys deposits an old suitcase at Paddington Station and then passes the cloakroom ticket to Miss Marple while she is lunching with Bunch at the Apple Bough. Gladys does all this willingly and reliably although it appears likely that she did not understand what she is doing.
Gladys is described as "a smart young woman with a lavish application of rouge and lipstick".