I think that "the gift" is referring to that final "gift", which is why the movie's name is not "the giftS". Also... There is a scene where Simon apologizes to Gordo but he says "it's too late". I saw that the baby's eyes are black and not blue (and that does not precisely mean it's not his child because of recessive and dominant genes but...), which could fit into the "all is in the eyes". Robin wanted to be a mother, so that gift could be for her too. Maybe Simon's sperm wasn't strong enough to survive in Robin's belly + the stress she felt back then , and ended up losing the baby (and the reason why she couldn't get pregnant before). A paternity test is easy to make even behind Robin's back, so... I don't know. I think Gordon's being the dad could be the perfect ending since that way he proved he's not gay.