There is really no indication in the novel that Pammy is Daisy's child with Gatsby. Even though Daisy says, "She doesn’t look like her father...She looks like me. She’s got my hair and shape of the face," she does not indicate in any way that the father is someone other than Tom (Chapter 7).
As well, Nick describes Gatsby's reaction to meeting Pammy for the first time, saying, "Afterward he kept looking at the child with surprise. I don’t think he had ever really believed in its existence before" (Chapter 7). The existence of Pammy is one crack in Gatsby's argument to Daisy that you can repeat the past. He wants for Daisy and himself to start back where they ended 5 years ago, but Pammy is a very real reminder that they cannot.