G.O.S.U = God Of Starcraft Universe
that means people who's got high skill in the game "Starcraft" (RTS)
Nowaday, it is still used to tell how impressived, skilled is a player in every game (particuliarly in Warcraft III actually).
Gosu players have skill, but have respect for other players and have good manners too !
OMG, with only your warden and some units, you kill his 2 heros and his army ! You're a gosu !