Daisy Buchanan: Rich girls don't marry poor boys.
Jay Gatsby: How do you do, old sport? I'm Gatsby.
Nick Carraway: They say you killed a man.
Jay Gatsby: Only one?
Jay Gatsby: Klipspringer has been here since a party I threw in April. I didn't even realise he was here until two weeks ago.
Daisy Buchanan: And when I was in the delivery room, waking up from the ether, I asked the nurse whether it was a boy or a girl. She said it was a girl - and I turned my head to the side and cried. And then I said, I hope she grows up to be a pretty little fool. That's about the best a girl can hope for these days, to be a pretty little fool.
Answer: I would say Gatsby knew about the child but suddenly realised she had a life without him and that she loved her children so her leaving was not going to be as easy as he imagined. Gatsby sees both Daisy and Tom in this scene being loving parents (for a minute or so) and it was hard to witness.