Only available to World of Warcraft Game Masters. The full set includes the helm, Gamemaster's Robe, and Gamemaster's Slippers. You can see them worn by an actual Gamemaster's character profile on the Battle.net Armory - Melesong@Illidan. (No one quite knows why this happened, but we know of it thanks to the World of Warcraft sub-reddit.) I uploaded a screenshot to Imgur.com just in case the profile is eventually removed. View it here.