Hi Mikamark,
One of my good friends that I walked with on the Frances is allergic to gluten and he had no trouble finding things to eat, and the Camino Portuguese will be the same. There are many shops and restaurants along the way where you can buy foodstuffs with no gluten in them, and all the big cities have supermarkets where you could buy gluten-free products, if you really want gluten-free cookies, breads, etc., but I must warn you those products were not impressive.
Adding to the fact that you are walking the most "urban" of the Caminos, just to be on the safe side, my friend bought a few extra snacks just in case there might have been an issue, but there never was, and he eventually stopped carrying the extra food, but you always can till you feel comfortable. If you do plan on eating at restaurants, let them know about your food allergies and they will be very accommodating, both in Spain and Portugal.