TRex is a Landry's restaurant. So while I would assume they wouldn't take a Disney Gift Card, they did ask me if I was on the Disney Dining Plan (and Rainforest Cafe asked me the same thing, also a Landry's place). I thought that was strange, since they are not owned by Disney, but may they have an "understanding"... I'd be interested to know.
T-Rex will accept the premium and platinum plans but not the DDP. They doubtless ask so they can confirm for you that you can't pay with the dining plan there.
When I called for my ADR's, (DDP) I questioned if T-Rex was on the list. My reservationist, said no, but call back after the New Year and it might be. Is that possible? DS really wants to go.