To make this Apple Fritter Breakfast Casserole Recipe gluten free, substitute the croissants with gluten-free croissants or gluten-free brioche-style bread. Confirm that the applesauce, vanilla extract, powdered sugar, and cornstarch are certified gluten free, as some brands may contain additives or be processed in shared facilities. The texture may be slightly different depending on the gluten-free bread used, but the casserole should still bake up rich, soft, and flavorful.