Hi, Carol!
Happy Birthday! I am a December birthday as well - how lucky are we? It's the best time of the year to celebrate! To have your birthday party in Disney is extra special, that is for sure.
Walt Disney World restaurants are very good about accommodating special diets. You can read more about it
HERE. When making your dining reservation, there is a place to make notations of food allergies. If you marked your need for a gluten free diet, you are in good shape. If you did not, give a call to Disney Dining at 407-WDW-DINE to add this to your reservation.
There will be gluten free desserts available, but for the cake, just to be sure, I would call the Cake Hotline to speak to them about what magical cake they can create! They can be reached at 407-827-2253. You might also want to visit BabyCakes in Downtown Disney (a vegan bakery) where they have gluten free cakes as well.
So many yummy options for a magical birthday celebration. Enjoy!