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 !
Night Elf is a versatile race, whose fragility is only handled well within the hands of a gosu player who can avoid the onslaught of piercing and siege on the race's characteristic unarmored units.
You: "...except natural light."
He is a gosu at Warcraft.