The Monaro exists only because GM owns Holden. GM bought Opel and Holden a long time ago.

All of these companies designed these cars after they were already under the GM corporate umbrella. It's just that now we're seeing a car that was originally developed by a GM brand outside of this country sold here under a domestic brand.

The Opel Omega was developed by GM for sale in global markets. In Australia it happens to have Holden Commodore badges on it. The Monaro is a mutated coupe version of the Commodore. Holden created by using a GM platform. Not a GM USA platform, but a GM platform nonetheless.

Hell this is not even the first time this platform has been sold in the USA. The Cadillac Catera is a gussied up Opel Omega.