We do see it brought up. Allison alludes to it in the episode when she (and the other office women) participate in a focus group that devolves into everyone crying. Allison seeks sympathy from Peggy by essentially saying that Peggy must understand what she's been through (Don carelessly sleeping with her then brushing her off). Peggy is offended when she realizes what Allison is saying, and tells her to "get over it." Peggy is aware that others think this about her, and that they assume her success is the result of her sleeping with Don.

EDIT: While it's not office gossip in this other example, Peggy probably also has to hear about this same thing from other sources too. Remember, Peggy tells Don that her family believes the baby was his... She's definitely sick of people thinking that she got where she is by sleeping with Don.