If you're self-catering then yes, you should be able to find gluten free products in the larger supermarkets. Here's an example of what you can get from Auchan, a chain with branches in Krakow:
Gluten free in Polish is 'bez glutenu' easy to say! You'll also see a lot of products with 'bez konserwantów' which means without preservatives and 'bez dodatku cukru' without added sugar. Hard to get things without salt which is 'bez soli'.