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.