No such thing as a factory 6 cylinder GT. The basic requirement for the GT option in 65/66 was a 225hp or a 271hp 4 bbl V8.
This would be a conversion. Ford actively promoted dealer installed parts to make any Mustang look like a GT. Virtually all parts were available as a bolt on so a car can look like a GT, but it would not be considered a factory GT.
What he said. IF for some reason it's a factory freak, I'd want to see documentation. Otherwise it's just a factory six with GT goodies slapped on later.
If you really want it to be a "GT" I would say run it through the GT check list ignoring the missing V8 option. I am guessing it will fail something else, you turn on the fog lights, the tail lights won't come on (tail lights coming on indicate factory wiring vs dealer installed).
Also, does it have DUAL exhaust with correct factory hangers? If manual steering, is it quick ratio? Disk brakes up front and labeled on brake pedal with the larger brake fluid jar? (No power brakes) Larger sway bar?
I am guessing if someone had a GT donor car with all of the GT goodies, they wouldn't have stripped it to put on a six banger.
The only 6 Cylinder Mustang EVER given the "GT" name would be a 1968 GT/CS the only way it ever happened. 1968 GT California Specials were offered as a 6 Cylinder. The High Country Special was also, but they did not call it a GT.
have you actually seen the car yet? it might be the sprint package with the six, and some uninformed people call it a GT. otherwise the GT moniker was never put on a mustang, except for the GT/CS as indicated.