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".