GSOS is an acronym for the phrase "Get shit on shitty". You say GSOS to someone whom you have owned or beaten, usually in a video game.
Pronunciation for this word is "g-sauce".
While playing Teamfortress 2 a soldier rocket jumps towards the medic. The medic then pulls out his Übersaw and stabs the soldier right as he hits the ground dealing 61 damage and killing him. The medic then taunts and says "GSOS".
Grab Stab Operations. "Grabbing" a girl (macking on her taking her home etc) out clubbing or partying. "stabbing" her (fucking her real good). "operations" a tactical way of going about this procedure. Originally created by two gentlemen by the names of Steve & Shea from Vancouver BC, Canada.
"i pulled some nice g.s.o's off when i went clubbing with the boys from east van."