Official WoW raids require 10-25 players. With No GCD, a single player can control a character that outputs the DPS of an entire raid. You can solo Icecrown Citadel (ICC) on Heroic 25-man mode because your Paladin can cast Divine Storm, Crusader Strike, and Judgment 20 times per second.
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In official World of Warcraft, almost every ability triggers the Global Cooldown. When you cast Mortal Strike, Fireball, or Flash Heal, all other abilities spin for 1.0 to 1.5 seconds. This ensures turn-based decision-making and prevents button-mashing. Official WoW raids require 10-25 players
A "No GCD" modification removes that restriction entirely. or Flash Heal