Mrs. Gunderson is a former resident of the building and lived on the third floor in apartment 3A. She was a neighbor of Penny, Sheldon and Leonard and lived opposite Mrs. Vartabedian. She has only make appearance in "The Lunar Excitation" (S03E23). There are often flowers on a plant stand by 3A, including poinsettias before Christmas. Judging by the wall behind Mrs. Gunderson, the floor plan of 3A is different from that of Sheldon and Leonard's apartment upstairs.
When Penny got drunk and spent the night at Leonard's, Mrs. Gunderson was able to hear them having sex, as she lives directly below them and they were making a lot of noise. The following morning when Penny heads to work, Leonard follows her down to the third floor and Penny tells him that them sleeping together was a mistake. When Penny leaves, Leonard shouts at her angrily and accuses her of using him for sex. As he makes this comment, Mrs. Gunderson exits her apartment. She and Leonard smile at each other and Leonard says good morning. Mrs. Gunderson says good morning as well, then jokingly says "Or, should I say, yee-haw?" a reference to what Leonard and Penny had been shouting during sex. She then winks at Leonard as he slowly walks away in embarrassment.
In "The Gyroscopic Collapse", it's revealed that Apartment 3A is completely empty. This suggests that Mrs. Gunderson either moved out or passed away.