Most of the Ford prep kit involves the factory drilling holes in the bed so the Hensley adaptor can be attached. That's the way it works for Ford and the Hensley adaptor I have. I did not get any kind of adaptor as part of the Ford option. Hensley supplied the adaptor and hitch. The adaptor mated up to the holes in the bed that came as part of the Ford prep kit.
Doesn't the GM prep kit option include drilling the holes ??
Call Hensley first and ask them directly if you still need the GM factory prep kit to get the holes in the bed.
Find out EXACTLY what comes with the GM 'fifth wheel prep kit'. SOMEBODY has to drill the holes.