If you're using a Marriott gift card, it SHOULD be processed just as if you're using another credit card. 80% of the time I have used one, it works just like a credit card. Just ask for a paper print-out of the bill after they use the gift card. In Hawaii once the person at the front desk tried to use the card as "credits" which would lower the total bill. I.E. less points for me, as the price of the bill was lower, rather than having it used as payment.

Basically, you're probably fine - just check the bill.