For gluten free products: the whole production chain needs to use different tools or be sealed off from the rest. You can generally use the same mill, kneader, oven, tray for barley, wheat, rye, etc without meticulous cleaning in between. But if you want it to be gluten free you now need to either do that expensive cleaning or more realistically have an entirely separate set of machinery and ensure it never gets in contact with your main line.