Sorry I'm not on your server but I'm sure someone from Shiva will read this
GG is in fact Good Game but mostly used in a sarcastic way when someone doesnt do/know the mechanic and misses to do it right.
"ig" is simply "in-game" where you oppe "rl" to aka "real life"
With nfc I cannot help you never heard/read this abbreviation :/

GL finding a good helper