AFAIK Honda does pretty much everything in house.
Honda automatic transmissions in cars are quite different to conventional automatics, they do their own PGM-FI instead of buying it from bosch etc etc etc. I think they worked with someone a little on SH-AWD, but even that was primarily honda with outside eng. consultation.
Since the new GS's gearbox is inside the engine cases, surely it's built in house by BMW. Maybe they outsource the gears, but I imagine they make many of the parts and certainly perform all the shimming/assembly into the engine case.