If I am out & about, Guzzy is nearly always my top choice for lunch or dinner. Their menu has fresh & delicious mexican choices, but the salad is usually my pick for a healthier option. Their website outlines clearly what is GF & outlines how cross contamination risk is reduced, although risk cannot be zero as they handle other things in there too.
Update: since this post, I've learned the fries now have a may contain statement (they never used to) due to changes on their manufacturing line. The fryer is not shared (but always check) but many coeliacs will not want to order the fries because of this. I've never had any issues, but some aren't comfortable with this risk so avoid the fries if that's you.