Gluten Free Blackberry Sheet Cake is spectacular! This fantastic cake is rich, decadent and absolutely heavenly. No one will ever believe it’s gluten free! This recipe has only eight ingredients including blackberry pie filling so it’s pretty easy to make. It’s a terrific dessert for company or holiday dinners, especially if you have friends or family with gluten intolerance. The recipe can also be made with a regular cake mix for those without gluten issues.