well take your room rate for the date of your trips and then see what 35% off per day is. If your room rate is $150 per night, you will save $52.50 per night for a savings of $262.50.
If you do the regular dining plan at $41.99 per night for adults, it will cost you $419.99.
So taking the $500 gets you an extra $237.50 in savings over the room discount.
The dining plan is irrelevant to your equation because the question is whether or not you will spend more then $419.99 on food during your trip. If you will, then the dining plan is a better option, if not then paying OOP is a better option.
Depending on when you receive the gift card (if you receive it before you check in), then you may be able to add the dining plan a few days before you check in, and use the gift card to pay for it at check in.