Holden is a company in Australia which is partly owned by GM General Motors in America. GMC is another part of GM in America.
Auto Companies usually do this in order to have specific customers satisfied with their services. For example, the cars in America may not appeal entirely to the Australia crowd. So the America companies open a new company, find a local partner and go ahead with business. This was, the Australia people will have their specific needs satisfied. Their desire for style, comfort, adventure etc will be met with Holden handling R&D in Australia and information flying back and forth to GM - General Motors