In the short term, PSA will just produce Regal and Commodore for GM as long as the check clears.
In the mid term, GM can source the Regal and Commodore sedan from China. The wagon is more problematic but the wagon business may not be big enough for GM to loose sleep over.
In the long term, I think there is no point for a Regal sedan in the US (it will continue in China). And Holden can decide if they want to base its Commodore on Chinese Regal or Chinese Malibu.
Holden's schizophrenic line up of part Chevy, part GMC, part Buick rebadges is a little weird to be honest. They should pick which design language they want to align with... Buick or Chevy.
Spark --> Chevy Spark
Barina --> Chevy Sonic
Astra --> Buick Verano (why not Cruze?)
Commodore --> Buick Regal (why not Malibu?)
Trax --> Chevy Trax
Equinox --> Chevy Equinox
Captiva --> Chevy Capitva
Acadia --> GMC Acadia (why not Blazer?)
Colorado --> Chevy Colorado
The Astra hatch is the Verano, but the Astra sedan is the Cruze (Just to make things even more confusing).