You will need to check with your TA to make certain they accept them and find out how to apply them to your account. I use DU and apply the card on their website.
Just learned about this for our upcoming cruise trolling on the disboards. not sure about DU but with our TA, costco, you just call in with your GCs and then they call DCL directly and apply them. I'm about 40 cards in as it I don't want any mistakes on the card numbers so I've been doing it in batches.
I called DU on the phone. It was faster to use gift cards. They will allow 10 gift cards per day. I used about 40 gift cards total to pay off our cruise over the course of a few months.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!