Gosu (고수) is a Korean term used to refer to a highly skilled person. In computer gaming the term is usually used to refer to a person highly skilled in multiplayer games like StarCraft, Warcraft III (RTS/RPG - DotA), Diablo II (RPG), Counter-Strike (FPS) and others. The term was adopted by gaming communities in many countries because of a large South Korean presence in online gaming communities.
The origin of the term is from hanja: 高手, literally "high hand", also meaning pro or highly skilled at something, is used for a person with skill, usually in martial arts or in baduk (the game of Go). In the dialect of the Gyeongnam province, gosu also has the meaning of "leader". There are several English backronyms of the word, mostly originating in the StarCraft community, including "Graduate of StarCraft University", "God Of StarCraft Universe" or "God of StarCraft Units".
Related terms 
Though not as popular, there are also several other commonly used Korean words for describing gamers with various skill levels. Joongsu (hangul: 중수, hanja: 中手, literally "middle hand") stands for "a moderately good player", hasu (hangul: 하수, hanja: 下手, literally "low hand") for "a mediocre player" or "a person with no skill" and chobo (hangul: 초보, hanja: 初步, literally "first step") for "a novice player". Hasu and chobo are the same skill level, but chobo is disrespectful or derogatory (whereas hasu is not). The English equivalent to hasu would be "beginner" and chobo would be "noob" or "lamer". Chobo is sometimes used by English speakers in the above disrespectful or derogatory manner, especially in the StarCraft 2 community.