leo, is the dad, ok i'm a year late but hey i just got a computer a few months back!!!!!!! lol

Grace and Leo separate because he cheated on her with another doctor while he was away in Cambodia, and months later when Will and Grace are on a plane to London to see Billy Elliot, Grace runs into Leo on the plane. They have a quick fling, and then decide to part ways again. A few months later Grace finds out she's pregnant. Grace finds out Leo is getting married to someone else, and she doesn't tell him she's pregnant, so Will agrees to be there for her child as a father figure... the Finale reveals what happens next :) That basically recaps it. Two years too late... but I still thought I'd help out :)