There is at least one that you can get gluten-free King Cakes, I know I get mine from a guy that is at Who Dat Cafe in Marigny neighborhood, which is just outside the Quarter. He always has different savory items and sweets as well. Talk to him, if you have special restrictions, he will go for it if he can (eg Nut Free, Soy Free, Dairy Free.) I go there regularly...