Is commercial masa used for making tamales gluten-free?

No. Cornstarch is the endosperm ground into a fine powder used primarily as a thickener in soups, stews, gravy etc, although it has other household uses as well. Masa is a mixture of flour and water used in many recipes. It can be made with various grains, but the most common are corn, hominy, and wheat. Tortillas are a very common use of masa, however there are myriad uses, mostly in Latin American cuisine.