Gluten is the protein found most commonly in wheat, barley, rye, triticale (a hybrid type of grain that is a cross between wheat and oats),
and most oats. Oats that are certified as
gluten free are to be treated as if they have
gluten. The explanation on that one is tricky, but it has to do with the peculiarity of oats to cross pollinate with grains that contain gluten.