Hi there, Alison!
The short answer to your question is sometimes. If you are on the
Walt Disney World website, you may notice that some locations, such as
Gaston's Tavern, indicates that it does not participate with the
Disney Dining Plan. While they do not have menu items that you can use your quick service or table service credits for, they do offer some menu items you can purchase with your snack credits.
Every menu will have a symbol next to the items that you are able to use snack credits for. If you're looking for something snacky and don't see that the restaurant is listed on the dining plan, just check the menu to look for the small purple and white dining plan logo next to items that you can use your credits for. And I must say, no one snacks like Gaston - the warm cinnamon roll is a family favorite in my family - and it's shareable!
There are a very few locations that do not accept the Disney Dining Plan. It's always worth checking something out before thinking you'll get a no answer.
Have a great visit on your upcoming trip, Alison!
Anna