Generally any other gluten free grain flour that is "white" (quinoa, rice, and sorghum were great suggestions, oat would probably work too) will work well. Don't sub brown rice flour or flours with more flavor or of different origins (I.e. Bean flours like garbanzo or vegetable/ fruit flours like potato, tapioca or coconut)

Rice flour is another substitute that works.

Sorghum or quinoa flour can be substituted for millet flour in gluten-free recipes.