These are nothing more than simple entertainment - there is no link between GMC and Rolls-Royce today, and both brands would likely be horrified by the results shown here.
But a cross-breed can be fun to explore visually: Simply because it is so outside the norm - so unlike anything you see on a daily basis.
In that fun and silly and non-speculative note, here is a rough, rough mix of Rolls-Royce Wraith and GMC All Terrain and Denali styling -- on one mutant vehicle.
The more I look at these, the less I want to publish them....