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.
2000 X-Star, 3000lbs ballast, LTR 330, Acme 1235 14.5X14.25prop.