You can't "stack" discounts with Disney, meaning you can't have to promos on the same reservation. However, if you are going for longer than the 5 days needed on the GC promo, you could do a split reservation, where the first half is for the GC, and the second is a room only with your PIN discount. Then, you can use your GC to do things like upgrade the length of the 5 day ticket, pay for the balance of your room only (after check-in), pay for dining during the second portion (you can't add the dining plan to a room only ressie).