I actually did call Disney about this a few minutes ago and the rep said it is not accurate to say your bill would be $0 on the Dining Plan. Because once they bring you the bill and you add the tip to it, which they do in the beginning of the process at Disney restaurants, you now DO have a charge on your bill - whatever the amount you decide to leave as your tip is now what you are responsible for paying and as long as it is a Disney restaurant that accepts Disney Gift Cards and that includes just about all of them, it is perfectly fine to pay what is left of your bill - in this case just the gratuity, it really doesn't matter, with a Disney gift card or whatever at that point.