used in online gaming, usually after killing or fragging an enemy, stands for "good game owned". meaning good game, but i am better than you, therefore i 'owned' you. variations include GG NEWB, GG NUB, and other misspellings of the word 'newb' or 'owned'.
Can also be used when someone else disses/kills/frags another person/player, as if to say, good game to you, but he just owned you. (seen in example 2)
leetcsplayer: HAHAHA GG PWNED YOU
notleetcsplayer: Yes you are so much better than me.
newb that talks a lot: haha i am so good u are all newbs.
leetcsplayer2: have a cup of GGSTFU.COM
leetcsplayer3: um gg pwned