Shanrrbananrr says GG69. she also calls her fans c*nts. so...
Well, I know pretty much... 0... people who call her the latest authority on MCSG slang, so.... yeah?
GG69 = Good Game - I have legit never seen anyone ingame use this, and not even on the forums or in Skype.
GG10 = Good Game 10 Hearts - Generally used when disrespectful. To piss them off even more, use GG12. (Golden apple/absorption)
Get Rekt = You got Pwned - Well, it's just a different version of "Get Wrecked" and is usually used in a disrespectful manner.
GG = Get Good - Depending on the context, this usually means Good Game. It is only used as "Get Good" after you completely wrecked someone AND want to piss them off even more (In other words, it's only Get Good if you're trying to be disrespectful.)
OMB = Oh My Bacca - WTF? WHO THE HELL SAYS THIS?
Also, there's
OMG (Generic term for "Oh my gosh" or "Oh my god")
WTF (Generic term for "What the f---")
Alt+f4 for diamonds/team/admin rank/money(Basically asking to get banned. Don't actually put this one in. xD Doing Alt+F4 will close your Minecraft client.)
m8 (Mate)
lolm8 (Lol, mate)
gg10m9 (Good game, 10 hearts mate - Generally either disrespectful or joking/teasing. M9 = M8)