About 15+ yrs ago a definition of a GT car was published in a racing magazine, I can't remember the whole thing but the bits I do remember are:
A maximum of one door on each side of the vehicle.
A maximum seating capacity of 4.
I remember that cos I joked back then about the Mini being a GT car, just a shame i can't remember the rest
And yes, someone did try (for a few rounds) to run a VW Golf in the British GT Series a few years back.