When you talk to Brenda with WDW Special Diets, she will make a note of the gluten issue in all of your reservations and have you fill out a form stating which people have the problem. If you want, she will also have each of the chefs call you at home. I love this. It really gives us a good sense of what they can and can't do for us, and in some cases, the chefs have asked us a bunch of questions about what we like and have made special things just for us. Last year at Akershush (is that how it's spelled?), the chef even made amazingly delicious gluten/dairy free chocolate chip muffins for my daughter. This most recent trip (February), since I knew we'd be eating many meals at the hotel, I asked to speak with a chef at the hotel as well. They were truly amazing about checking into things for us and even ordering in a few things that they don't normally carry.
Almost every table service restaurant that we've eaten at has both gluten/dairy free brownies as well as gluten/dairy free ice cream. We've never managed to get a reservation at Le Cellier though.