GM event is a holdover from EverQuest and the early days of EverQuest II, when a GM (short for "Game Master") ran live events, a special quest or task the player has to accomplish. In current usage, volunteer players called guides run random live events, while GMs are the paid employees of Daybreak Games who can push real buttons to accomplish things like banning players, unsticking characters, and completing gated quests.
Upcomming guide events are posted at EverQuest II Guide Events