KGB ballast is Keeline Gravity Ballast. It is as mentioned your ski locker sac/belly sac/center sac whatever u want to call it. Mastercraft branded that name in the first ever produced X Star with a single ballast bag in the locker. It just stuck. For a while KGB ballast was standard and the rear tanks were optional. Usually 375-400 depending on boat.
CURRENT: 2008 SANTE 210
2000 Mastercraft Xstar
2004,2005 Ski Centurion Avalanche C4
2003, Ski Centurion Cyclone
99,2000,2001, 2002 Ski Centurion Elite V/Hurricane/lightning
88 2001 Nautique