In Selden's apartment at the Benedick, Lily and he discuss his cousin, Gerty Farish, who lives alone and whom Lily considers to be unmarriageable.
Gerty is present at Jack and Gwen's wedding. She's in a great mood because Selden brought her as a date and is taking her to dinner after. She reveals to Lily that Percy is with Evie.
Gerty attends the living portrait party with Selden. She and he discuss "the real Lily Bart," whom only they know well. Gerty thinks she can bond with Selden over their mutual admiration for Lily.
The next night, Selden has dinner with Gerty. She has high hopes for a romance between them, but soon realizes that all he wants to do is talk about Lily Bart, with whom he is clearly in love.
Gerty rages (internally) against Lily but humors Selden by praising her qualities. After he leaves, she is left alone to muse on her growing hatred for Lily.
When Lily shows up, however, miserable and asking for help, Gerty forgets all about her anger and takes her in.
After Lily is disinherited, Gerty and Carry are the only two friends left to help her. Gerty feels she has a "moral claim" over Lily for having saved her that night after the Trenor incident. She encourages her friend to maintain her dignity and not go slumming with those below her class.
Gerty discusses the Silverton sisters with Lily.
Gerty meets with Selden to discuss her concern for Lily (and also because, hey, it's an excuse to hang out with her crush). She gets him to go and talk to Lily.
Gerty is already at Lily's place when Selden arrives the morning after her death. She reveals the news to him and tells him that he ought to be alone with Lily.