Disney gift cards are accepted in the actual parks, of course. In Downtown Disney, you can use Disney gift cards at World of Disney, Marceline's Confectionery, D Street and at Anna & Elsa's Boutique.
You can probably also use something like a Visa, MasterCard or similar gift card in the 2 parks and in DTD. I think that Starbucks gift cards can be used in the parks (at the 2 Starbucks locations), but just without being able to claim any of the Starbucks rewards.
Downtown Disney, on the other hand, is a whole different ballgame. Yes, the Landry's cards can be used at RFC. There are no other Landry's restaurants in Downtown Disney. You can use Earl of Sandwich gift cards at Earl of Sandwich, of course. You can use Jamba Juice gift cards at Jamba Juice, Haagen Dazs at Haagen Dazs, Build-A-Bear at Build-A-Bear, etc. .
The Patina Group owns a few of the restaurants in Downtown Disney -- Tortilla Jo's, Naples, Catal and Uva Bar. I don't know if they have gift cards available. I also have not checked for Ralph Brennan's Jazz Kitchen gift cards or La Brea Bakery gift cards.
At the Starbucks in Downtown Disney you can use a Starbucks gift card and also get the Starbucks Rewards (if you belong to that program), but you can't use a Disney gift card at the DTD Starbucks. Only the 2 in-park Starbucks take Disney gift cards!