We went on vacation to Girona and made a must-stop at Konig Vilablareix because it's the only one that offers gluten-free cuisine. I recommend the bravas, chicken fingers, and croquettes. The burgers (any) are very good.
You can't book, but we went on a Friday and got in pretty quickly and were served right away. If you have celiac disease, I recommend it, and the price is also superb!