Finally got a person that REALLY knew what she was doing. Took about 10 minutes to add in the gift card to the booking. The wait time was still long however to get someone. I tried the call back feature also. My experience on the call back is that it doesn't get routed to the rep, so therefore it hangs up and now you have to dial again and put yourself back on queue. Anyways, here was the process for anyone's reference, the challenge really is getting someone that's familiar with it:
1. Purchase the gift card at AARP. You should get a digital gift card in less than 30 minutes via email. The gift card has the card number and PIN. NO EXPIRATION DATE is needed.
2. Call 1-800-774-6237 choose 2, then 1. For me it was an existing booking to add the gift card as onboard credits.
3. Cross your fingers you get a rep that's familiar with the process. Tell them you would like to use a gift card for this booking for onboard credit. They'll take your card info and email you a confirmation of the onboard credit purchase.