Even if you purchase the gift cards with your Amex card, they are not covered by the travel protection. (I checked for American Express Platinum's standard coverage.) Only the part paid by the AX card is covered. There is supplemental coverage you can add which would cover it.
If you cancel your cruise, the $$ go back to the gift card, which is why you need to hold on to the original cards / PDFs.
For close-in trips, we pay with our Platinum Amex. We do occasionally backstop with HAL cruise protection. That's because "something happened to a relative and we can't travel" requires cancel for any reason coverage.