Technically it was originally an Italian designation for "Gran Turismo" which was a specific genre of racing in which the car was suitable for both street and track use, with the track being typically a long road course. The "GT" designation was initially applied to Italian sports cars that were meant for tract use but were also street-legal and actually fairly suitable for street driving from a comfort, practicality, and reliability standpoint. The "GT" designation was later appropriated by other manufacturers to designate the more sporting versions of a model.
"Gran Turismo" translates (rather obviously) to "Grand Touring" in English.
Black 2006 Mustang GT Premium Coupe | Interior upgrade package | 18" 'Torque-Thrust' style wheels | Shaker 1000 | T3650 | Side airbags