I'm gluten intolerant but handles traces. And I found it really hard to find food (but I have some other allergies as well complicating things). Tuna salad will always work in the bars but the French fries are fried in the same oil as croquettes and other bread based things so ask or check menu carefully. The fish is often panned or at least rolled in wheat flour
Meat without sauce is normally without flour and soups but you have to ask. Tortilla is mainly made without flour but as I always ask I sometimes is denied to buy it.
Not all stores carries gluten free things as bread and pasta but larger stores normally do. Restaurants are better than bars to have options for gluten free.
Of all the routes I have done Camino Francés was at least the easiest one when it came to food. They are more customed to us than on other routed with fewer pulgrims.
If I carries bread I asked for a plate of ham and cheese in the bars and made my own sandwiches. On this camino (Norte) my breakfast was often nuts and baby bell cheese.
Get a card with your intolerance written in Spanish. I got a friend to translate for me as I have other allergies as well otherwise there are some cards to find on the internet. Learn a few phrases in Spanish. Sometimes they don't understand sin gluten as they don't know what gluten is but celiaci then know about.
Buen Camino and good luck with the food.