Watch out for mayonnaise. This is one thing that nearly always contains wheat in Spain, yet most employees at restaurants and bars (especially those that do a high volume of tapas) are not aware of this.
Most Spaniards have heard of a gluten-free diet and are aware that celiacs are allergic to gluten; however, I've found that a lot of people don't know what gluten is. It is very important here to specify that gluten is found in wheat (trigo), cebada (barley), and centeno (rye), which means no wheat flour (harina de trigo), no bread crumbs (migas), no bread (pan), or mayonnaise (mayonesa).
A great gluten-free place in Granada is El Piano. It's an organic, specialty, fast-casual restaurant that specializes in special diets. Everything they sell is gluten-free and safe for diabetics. Plus, you can get a full lunch with drink and dessert for 5 euro.
Walking on the Camino Frances (St James' Way) - El Camino de Santiago
I made the Camino three times. In 2006 I could hardly find anything for coeliacs, but two years later I was surprised to see the GF sign on lots of products. The Spanish have a lot of coeliac groups and societies in many towns and can provide you with information. See the list here: http://www.beiker.es/enlaces.php Unfortunately, not many speak English.
You can find a few GF products in most supermarkets, the best of which is SIMPLY Market full of GF products (bread, muffins, pasta, cakes, etc.)on separate shelves. The sausages and cold cuts are not sepatated, but most of them are GF. There are also also quite a lot of GF salads and dairy products available. I came across the first Simply Market in Estella.
If you go to a restaurant, take the gluten-free restaurant card in Spanish http://www.celiactravel.com/restaurant-cards.html and show it to the staff. They will help you and will do their best. Luckily, the Spanish don't use flour very often, but they have some breaded dishes, so warn them not to fry your potatoes in the same oil. There is a little village called Ages whre you can have a GF menu in the second albergue. See the map here: http://caminodesantiago.consumer.es/etapa-de-belorado-a-ages
All in all, it's getting easier to keep your GF diet in Spain.