Since every VIN matches it sounds like a factory VIN mistamp, there is no "GA" body code.
There is a GA interior code in 73 though which is the black Mach1 interior. If the car is a fastback and a true Mach then the body code should be "05". A worked stamping the VIN may have screwed up.
This is why you can't get it to work in the Secret Decoder. The Secret Decoder does have another problem too with some real VINs but nothing that would affect your car or 99+ % of others either.
__________________
1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982.