What effect does Daisy's daughter have on Gatsby?

Gatsby seems short of shocked when he meets Pammy, Daisy's young daughter. Daisy greets her child, crooning to her and encouraging a hug, and then the little girl meets Gatsby and Nick, shaking each of their hands in turn. Of this moment, Nick says,

Afterward, [Gatsby] kept looking at the child with surprise. I don't think he had ever really believed in its existence before.

Despite the fact, then, that Gatsby knows Daisy has a daughter with her husband, Tom, Gatsby still seems surprised when faced with proof of her actual, physical person. Gatsby is really adept at living within his imagination: for all these long years, he's convinced himself that one can absolutely relive the past, that it is, in fact, possible for him and Daisy to return to the people they once were before she married Tom. However, the fact of this child's existence kind of ruins that fantasy, because, even if Daisy were to leave Tom for Gatsby, Daisy's daughter would, in all likelihood, come with her. This means that it wouldn't just be the two of them—Gatsby and Daisy—as they were before the war; Pammy would always be a reminder of their time apart and the things that did, indeed, permanently change in Daisy's life as a result of her marriage.