GlobalCooling dims the cursor icon during the global cooldown. Due to limitations imposed by Blizzard this only works if the mouse is not hovering over the WorldFrame.
Use /gcool <SpellName> to set the spell it should use for detecting global cooldown duration. It is advised to use a spell that cannot be spell locked and doesn't have a cooldown like "Find Minerals".
The entered spell is saved on a per-character basis, so you have to set it up once per character.