Game master
From Guild Wars Wiki
A Game Master is an in-game administrator that works for ArenaNet or NCsoft who can interact with players in-game, make announcements, ban and suspend accounts and tag accounts with an account mark.
Messages from GMs appear in a teal color in the chat. This was changed in an unknown update, before teal the color was purple, and before the March 2, 2006 update, they appeared in green.
