Gertrude, possibly Gertrude Baxter, was a human woman.
Gertrude was a slightly older cousin of Martha Ravenswood's, who spent her life stuck in a menial job in the Thunder Mesa saloon, cleaning it up every night. Her dismal example discouraged a young Martha from seeking an honest job, much like how her older sister Mary's experience with marriage dissuaded Martha from seeking a husband (until, that is, she met dashing robe Henry Ravenswood).
Gertrude was mentioned in the unofficial 2020 story Enchantée. Her last name is not mentioned, although if she is the daughter of a paternal uncle of Mary's, it might well be the same as Martha's maiden name as posited by Enchantée, “Baxter”.