GAT is renown here and a lot of people use them to get meal vouchers that rival AP discounts especially when buying a lot of tickets and vouchers at once. I would say that their character meal voucher for 3 or more tickets equals a premium passholder price and might be a few pennies more than other AP rates or less depending on amount you are buying.
Someone from that site posts a lot here so they know when there is trouble brewing with the site and I rarely if ever hear anything bad. My warning is to shop around to make sure you do get the best rate. As a premium AP buying one voucher I would pay way more but for a tourist with no AP and buying 3 meal vouchers for 4 family members then I would save money big time.