It's a matter of your stance on what constitutes "not authentic". A greymarket shoe is a shoe made with out the authorization of Nike. A true GM shoe is made with the same materials, and the same factories but sold through the back door. A lot of these shoes don't go through nikes quality control so quality can range drastically between pairs. Some people call them fake because they're not authorized by Nike, some people see them as real cause they're made from the same materials. Although these shoes are distributed through the same channels as replicas keep in mind that they are NOT the same. Replicas are crappy/lesser quality versions of the official shoe while GM shoes use the same materials.
Not every shoe has a GM version but many recent releases do. Some shoes that have been labels GM aren't really GM but actually fake like the DB4 and Royal Blue 1s. These have been proven to not be made of the same materials and can even have different shapes and constructed differently. Hope that explains it.
