I used to have a GM card at the old 4% earning rate. It was probably the best I had, certainly in those days. By specifying a Vauxhall as a company car, I was able to select other rewards rather than a discount on a car. From a wide selection, I chose travel vouchers from Going Places - or was it MyTravel - anyway Thomas Cook now. I still have well into 4 figures and will probably use them for flights or a package.
That was then and I believe that option no longer applies.
Taking Teece's and dundalkspur's points, we still don't know what will happen to GM Europe. Assuming they are bought and continue to operate, I imagine the credit card will continue through Vauxhall dealers as before. It no longer suits my purposes, and I'm over-carded anyway, so I won't be bothering to apply.