'GM me' means 'gamemode me'. Players of Minecraft will say it (in chat) to the player whose opped (in a server) to change their gamemode as only the opped player(s) can change it. There are 3 gamemodes in Minecraft; Survival, Creative and Adventure mode. Generally 'gm me' is asked when a player wants to change to creative.
by Meinkraft Player August 10, 2013