There are a plethora of gluten free grain/seed options available and they include: rice, corn, quinoa, amaranth, buckwheat, tapioca, millet, chia, flax, potato flour, chickpea flour, lentil flour, coconut, and nut meals e.g. almond or hazelnut meal
Whether you need to eat wheat or gluten free, it’s best to mix up sources of grain/seed/flour products for reasons of variety, and most alternatives have better nutrient profiles anyway. The most nutritious options are: quinoa, amaranth, millet, chia, flax, coconut and lentil.
As per my previous post, I don't advocate the consumption of a lot of gluten free substitutes, however there is a place for them and understanding how they can be used from a flavour, textural and nutritional point of view, is important.