The problem is a large portion of the food in restaurants is not prepared in the restaurant. If you order a burger meal, yes the meat is cooked on the grill at the restaurant, but the buns, and any sauce were likely made somewhere else. They might purchase the meat pre-seasoned from the food distributor. And the french fries likely even have a seasoning pre-applied to them before they are delivered to the restaurant.
You have the ingredients on a package to check. The head chef can do it while constructing the recipe. And you can explicitly order gluten free substitutes that stay in the freezer until needed.
