All grains have glutens in them - not just wheat which has a particular type of gluten (gliadin) - it's just that wheat is particularly troublesome for people with gluten intolerances.
However, my understanding is that after wheat, corn gluten is the next biggest problem for many people with gluten sensitivities, especially if you have poor digestion or leaky gut syndrome. In fact, many people with gluten allergies, intolerance or celiac continue to have problems after they remove all wheat from their diets - and it's possible for some that it's because of corn gluten. I personally react as strongly to corn as I do to wheat (perhaps more so). As a result, I seek out gluten-free baking powder which does not have cornstarch in it. Baking soda (as stated previously) does not have either wheat or corn gluten.
So, there is in fact a reason for gluten-free baking powder.
The "Gluten free" on a baking powder label has nothing to do with corn. It could still have corn starch. For labeling purposes and Celiac Disease - corn " gluten" is not called gluten.