Game Master

Game Master or GM is a generic term that refers to an individual who acts as an organizer, officiant for regarding rules, arbitrator, and moderator for a DMMO-RPG.

Background
In YGGDRASIL, GMs were responsible in enforcing the game's rules and providing general customer service amongst the players.

The Undead King Arc
At 23:55:48, less than 24 hours before the shutdown of YGGDRASIL, Momonga awaited the end of the came. He thought of the GMS who probably were handling the public channels and setting fireworks. After the sadden player doused off, he found himself still in the game, and believed that the GMs delayed the server after checking the time. However he expected there to be announcement as they would have done so in this situation.

Overlord Blu-ray 02 Special Arc
During their time off given to them by Ainz Ooal Gown, the Floor Guardians gathered together discreetly to discuss about their creators and Ainz Ooal Gown. The main topic were the reasons for their actions and snippets of Ainz's private dialogue. Albedo mentioned about her hearing Ainz asking about the GM Call, but was unable to understand its meaning and was embarrassed by her lack of knowledge.

The Dark Warrior Arc
The GMs were mentioned by Ainz Ooal Gown when he thought about his adventurer gear and YGGDRASIL items. There was once a problem in the past, where the names of magic items made by players were written with forbidden language, or that it might insult someone. Sometimes, the GMs would intervene in the situation, warning the players in question to rename the items.

Strength
GMs were responsible for administering the laws of the game. In addition under the Social Order Maintenance Act, they had the duty of disciplining players to prevent sexual acts of conducts within their servers and were on the watch for lewd actions. Items of custom made by player were under their purview. If an item possessed an inappropriate name, the GMs would question the culprit and rename it. The GMs could be summoned through a GM Call which could be accessed via a player's console.

Trivia

 * In Hack And Slash Adventure, Ainz played the role similar to a GM for the Floor Guardians.
 * Originally, you could Shout or use a GM Call and immediately contact a GM.