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.
Shorthand for Get My Fuck On. On is pronounced "awn."
Your little sister's in town; time to GMFO.