has a gluten in it
(referring to oats)
Oats themselves do not contain gluten. HOWEVER: Oats can have other grains, like wheat, barley, or rye, mixed in with them, in tiny percentages, or are processed on the same machinery as the other grains. Wheat, barley, and rye DO contain gluten, which is a protein.
Thus Quaker Oats was able to market "Gluten Free Oatmeal" at a higher price.