Oat flour is an inherently gluten-free flour made from whole oats ground into a fine flour. You can replace up to 1/3 of wheat flour in a recipe.
You can may notice some oat flour products are labeled gluten free and some are not. For example, with Bob's Red Mill sells 2 versions of oat flour. One is labeled whole grain oat flour and another whole grain gluten free oat flour.
The reason for the difference is that the one marked "gluten free" is produced in a portion of the facility with equipment that does not share lines with their other wheat production products. Bob's Red Mill says their whole grain gluten free flour can be used 1:1 to replace all-purpose flour in a recipe.
If you have some rolled oats in your pantry then you can whip up your own oat flour (not just an alternative):