There are plenty of places to eat (sin gluten) just make sure they understand about the cross contamination - Richards Tavern on the way to cap Salou is good. Portaventura are also very good about sin gluten, we ate in the wild west area just over the iron bridge, they had a full menu just for gluten free but you have to book a time slot. also most of the supermarkets sell sin gluten larger it's about 4-5 euros for six bottles or around 1.30 euros for a single bottle. If you venture out into the old town there is a supermarket called Bonpreu it has a good selection of sin gluten products much better than any uk supermarket.
Mercadona has a good rep as a 'sin gluten' supermarket. The Owner's Son is gluten sensitive, and he has made a point of selling as many 'sin gluten' products as he can find to stock their shelves with
Also Route 66 restaurant in Cap salou will do their level best to provide sin gluten meals; you only have to ask Ellie or Sue, and they will help.