If you put in a gift card and it accepted it then you don't have to worry about billing. The billing address is often the same one they use for shipping, enter it and don't worry about it the gift card will take care of it.How to Buy online on Barns and Noble.com with a gift card?
This is for their protection. If your gift card turns out to be stolen or counterfeit, they can send a bill to this address. They can't send a bill to the shipping address because that person could say "it was a gift".
Since you have a real card, enter your address.