Out of curiosity I did a little research. I researched Subaru's parts database website. You can click on every part in the car - and then click "Interchange" to see what other vehicles use that exact same part. Overall - every major GT transmission part I found was the same as OBW transmission parts.
If you wanted to check every part and verify, go here:
My best guess is the transmissions are entirely interchangeable. The main difference in the Legacy GT is the "Sport-Shift" function. But I'm almost positive that's entirely an electronic capability and not a transmission-specific function.
The inhibitor switches (selector on the transmission) is the same part as other 4 cylinders and 6 cylinders, so "Sport Shift" is not anything mechanical in the transmission as far as I can tell.
I checked Subaru's parts database and the final drive ratios are both 4.44 final drive ratios.
Torque converters, valve bodies, reduction gears, planetary sets, ring and pinions, the complete overhaul gasket kit are all the exact same part and part numbers - so GT transmissions are nothing special and basically the same transmission. I obviously didn't go through every part but I did go through all the big ones - and they're all identical to 2.5 (and often even 3.0 H6) transmission parts.
Most likely it's just some slightly modified code in the transmission computer.
It might mean you don't have "Sport Shift" mode any more if you use an OBW transmission in a GT. But who cares about that.
The final test would be to compare wiring at the TCU or trans connectors and see what's different. More than likely they're identical and the only difference is the TCU.