Who invented the gym?

No one knows. The ancient Greeks had former champions from their own cities help them train for the great national Games (the Olympic Games, the Isthmian Games, etc.) The Roman gladiators trained full-time in a school called a ludus. The first modern American pro athlete to use what we now think of as sports training was Carl Yastrzemski, who in the winter of 1966 decided not ot get an off-season job and instead hired a trainer and worked out all winter. Most pro athletes do that now.