Definition

A person who manages, controls, and narrates the events within a game is often referred to as a "game master," or "GM" for short. A GM is responsible for setting the scene, presenting challenges, and assuming the roles of various non-player characters (NPCs) interacting with the players. In these contexts, the GM must be creative and quick-thinking as they adapt to the unpredictable actions of players and attempt to enhance the gaming experience.

Origin

slang.net