You can use the gift card at any Landry's owned restaurant (which T-Rex is). The back of the card says: This Landry's Restaurants, Inc. (LRI) gift card may be applied toward the cost of food, beverage or retail merchandise at any LRI restaurant.
A PP asked if you can make reservations at T-Rex. You can make a limited amount of T-Rex reservations through Disney either by calling them or going through their online reservation system. Or you can call T-Rex directly as they have more availability for reservations than Disney does. Their number is (407) 828-TREX (8739). We needed an 8 p.m. reservation for one of our dates in December but Disney only had 10:30 p.m. available (really?? who's eating dinner at 10:30 and would there really be a wait anyways during a weekday?) so I called T-Rex and got the exact time I wanted.