A shorthand version of the name
Gary Motherf****** Oak. The name is used to represent a rival who one-ups you at every conceivable instance, as Gary Oak did to Ash (or as his counterpart did to the player character in Pokemon Red,
Blue,
Yellow, Fire Red, and Leaf Green).
You just defeated
Lance, with only one pokemon left and no more revives. You're ready to be declared Champion, until
Lance reveals that someone else has already
beat him: GMFO.