I've been baking gluten free for a couple years now. Here's a gluten free, corn free, banana muffin recipe that I use all the time. The basic recipe includes 1 1/2 cups gluten free flour.
When I first started baking gluten free, I would use a mixture of 1/2 brown rice flour with 1/2 white rice flour. But I'm moving towards something a little more healthy. I keep this 1/2 brown rice and 1/2 white rice flour mixture all ready mixed together in a big bin with a lid with my usual baking ingredients. For this muffin recipe I typically take 1 cup of my pre-mixed rice flour and add 1/2 cup of an alternate flour. The alternate flours I use include: millet flour, chickpea flour and coconut flour.
One other tip is to use baking soda only, not baking powder. Baking powder has corn starch. If you are avoiding GMO then avoid using baking powder. I find that using guar gum with baking soda gives a good rise in gluten free baking.
1 1/2 cup gluten free flour (1 cup 50/50 white rice to brown rice mix plus 1/2 cup alternate flour)
1 cup sugar when using chocolate chips OR 1 1/2 cups sugar if no chocolate chips
1 tsp guar gum
1 tsp baking soda
1 cup chocolate chips (optional)
1/4 cup olive oil or yogurt
4-5 mashed banana
Mix all dry ingredients in medium size bowl.
Mix all wet ingredients in large bowl.
All dry ingredients to wet and mix.
Line muffin tray with muffin cups. Bake at 350F for 20 minutes.