This is an abbreviation for "Okay, i'm done fucking socializing with you now leave me the fuck alone before I leave golf-ball sized dents on your cranium."
Used commonly in internet conversations to politely end an argument.
<TehR0x0rZ> Hey. Do u like dogs?
<Person> What? GTFO, go hang upside-down from a vent and have your baby sister play piñata with an aluminum bat while a monkey repeatedly shoves it's disease-infested feces down your esophogus. K'thx.