Knowing that corn is gluten free, I have been using Maseca brand Masa Harina as my go-to flour when baking or making a roux.  There was no mention on the label of it being milled in a plant that also processes wheat products, so I figured I was safe.  After a recent issue with finding safe corn meal, I realized that I needed to investigate whether or not it was really safe.  That lead me to their website and this statement:

For instance, corn is naturally gluten-free, and when choosing a type of tortilla, select tortillas made with MASECA® (which is 100% selected corn) and your tortillas will be gluten-free.