During the credits, Trey and Matt (creators) have a "GF" symbol next to their name to likely make the indication, or joke, that they are now Gluten Free
The idea for the episode came from Trey Parker and Matt Stone noticing how people they work with, and society in general, were going on gluten-free diets. It became so common that they went on diets themselves and thought it would be fun to do an episode mocking themselves.