Why does Gatsby view Daisy's child with surprise?

A child is often a product of love, which he hopes is not really going on with Daisy and Tom. A child means at least 18 years of permanence between a man and a woman even if they separate.

Daisy also lives a life apart from her child. He's seen Daisy in many situations, but not ever with a child. This child suggests Daisy is not just a wife to be separated, but a mother. I think if there was any moral bone in Gatsby's body a child would give cause to think twice about what he intends to do.

I think he also would have liked to contribute to the children that Daisy would have.

Nick says that Gatsby likely never believed in the child's existence. The reasons above, to me, are likely why.