Hell; we might as well start calling people 'purple.'
Anyway; the question has been answered. Usually, people have no pigment in their gums. Some people, however, do. Congoid people, and other darker-hued people, have more pigment, so it's more noticeable when their pigment does overflow in their gums, or underneath their fingernails. Having dark spots in your gums is no more prevalent in coingoids than it is in caucasoids, or mongoloids... It's merely more noticeable.