I'm thinking it might be too late since the SS (coming tomorrow) has already been charged to the credit card. Apple would have to accept a return, refund the card you used, then apply the gift cards to a new purchase of the SS.
It's really not a gift card question at all. You can cancel the pre order and they won't be charged. The question is whether they will let you change the payment after you've received your watch. I'm 99% sure you missed the deadline for that by about a week...