Hi Reuver,
As you may (or may not know) I have not walked the camino, but I have lived about a year and a half in Santiago de Compostela.
In the food stores in Galicia, you will find some gluten free food in the "health food section" of the store.
My worry is that when you are walking in smaller villages, that you might have problems finding this kind of food.
I don't know much about your illness, but can you eat for examples potatoes, eggs and some onion? If so, this is part of the receipt for my favorite Spanish food. La Tortilla. You will find it everywhere, and it is quite filling. Alternatively would be a more vegetable diet. Not sure.
In the biggest food stores I think you will find the gluten free food, in the smaller ones maybe no.
Buen Camino,
Ivar