An early '65 GT would have simply had the same axle bumper plate as a K code. The rear build-up of the car where the dual exhaust reinforcements would have been carried over from K codes to A code GT's.
The first dealer price list to include the GT option was the second price list printing for '65 and had "April 1965" on the top. Dealers had to receive these sheets before they knew the new options were available. I suspect that the price sheets made it to dealers around late March '65. If you look at the Jim Smart/Haskell In Search of Mustangs listing, there are just a handful of cars listed as GT's with February dates. I suspect those were most likely GT clones as the authors had no way to verify every car. GT cars appear with more consistency starting at the end of March '65 and then going forward.