I am celiac and walked the way this past September. I had no trouble finding sin gluten food in the supermarkets. I did carry Glutenfreeda instant oatmeal packets for my breakfasts (12) and a couple of freeze-dried pouches of gluten free backpacker meals by Alpineaire to use in the event I was stuck for something to eat. They came in handy. I also brought a bag of Kilimanjaro mix by Prana for some trail mix I nibbled on as I travelled. I tended to eat things like chorrizo, serrano ham, cheese, fruit, veggies, nuts, dried fruit and chocolate. Lays chips are sin gluten
In medium to large grocery stores you will find plenty of food to accommodate your needs. Generally one packet of ham or chorrizo and one of cheese would do me for two meals. I also had with me this card
http://www.celiactravel.com/cards/spanish/ to present at a restaurant. It explains clearly what is required for a gluten free meal. I did find places that could accommodate me and others I walked away from because they did not inspire confidence. I never got "glutened" once. Even if the basic recipe is gluten free, I stayed away from the tarta de Santiago as the risk of cross contamination in a bar/café/restaurant is way too high. I did pig out
on one that I bought at a shop in Santiago that was labeled sin gluten. It was sooooooo good.
The shop is just off the Praza Cervantes. If you put the Cathedral behind you while facing the bust of Cervantes, it will be just around the corner from the left-hand colonnade on your left side. I posted the address in another thread. I will look for it.
Edit: The shop where I found the sin gluten tarta de Santiago is at 1 Rua da Algalia de Arriba