Take note that the expression gluten here is inexact; there is no true gluten in corn, but simply corn proteins. A misuse of the term by the corn industry has become common in recent years. It has become fairly common to call corn storage proteins corn gluten.

From celiac.com:

When it comes to celiac disease, a similar corruption of the term has become very common. There are certain related proteins in wheat, rye, and barley that give rise to particular peptides during digestion that are capable of triggering the responses typical of celiac disease. Only in the case of wheat can these be strictly considered to be derived from the gluten proteins. But for lack of a suitable term, patients and their physicians began speaking of gluten-free or gluten-containing foods. People ask me, How much gluten is there in quinoa? I have to translate this into, Are there any harmful peptide sequences in the proteins of quinoa? There is nothing in quinoa that is like gluten prepared from a wheat flour dough, which has an unusual, perhaps unique, viscoelastic character.