The whole deal is still a bit dark surrounding details. Apparently there is a clause dealing with partners not allowed toi sell cars developed by the other in markets where they compete or are not sold yet, and that it is indeed brought down to which company made the major part of development on which platform. I know the smaller cars are GMDAT in Korea, but either the Astra or Insignia were an Opel development, and I guess the car not developed by them will die rather soon. The death of the basically finished Opel Corsa is a clear indicator of these rumors.
A few years ago, when back the GM and PSA agreed to co develop cars (the Opel Crossland X is the first result, Grandland X soon the second) had seen small cars move to PSA's EMP2 platform and the larger Citroen C5 (and presumably the Peugeot 508) to become clones of the next Insignia, and being built by Opel in Germany. Though the large car deal somehow seemed to be canceled, I wonder whether it might happen after all never the less. Either way, Insignia or the Buick might have a short life, and the "no sedan for the US" announcement makes me think that probably the Regal will be on the loosing end.