I feel that this comes down to poor employee training as I had to process gift cards once for a masterbilled group booking and everything went through just fine. Unfortunately, due to discrepancies in training, I would say to present the gift card at check-in and have you account credited for the amount of the gift card in addition to receiving a receipt for the amount of the credit. Keep this receipt for the duration of your stay and present it at checkout if need be. I don't think Marriott Int. Cor. can be blamed so much for this as the "hands-on" training provided by the individual properties themselves.