GTO was a name that was John DeLorean's idea, inspired by the Ferrari 250 GTO, a successful race car. It is an Italian abbreviation for Gran Turismo Omologato
The GTO was an optional package on the intermediate-sized Tempest, called the "Grand Tempest Option, for 1964 and 1965, but from 1966 to 1971 GTO became a separate model.
It was referred to as a "Muscle Car", and also as the "Goat". Seems to be no agreement on why "The Goat".