Clearly mark on the menu what contains gluten. I had a gluten free pizza which they mentioned when I ordered that there may be some cross contamination (I am gluten intolerant so this wasn't a problem). It was a really good pizza although it did cost ~£4 more than a regular one. There are also others things on the menu that didn't contain gluten.
— Not very
— Gluten-free items are marked on the main menu
— Some
— Good