Ian, I have the GPS front and rear. The front is terrible on anything but the best gravel, it washes out around corners with very little grip. The rear I asked to be set up in the off-road direction and it is perfectly acceptable for the type of tyre it is (think Mitas E07). As a general tourer the rear is a good tyre.
As a positive road noise is very limited.
My experiences don’t relate to Namib sand, rather KZN and Eastern Cape gravel, mud and rocks. Watch your pressures, the GPS can slide around a bit if pumped too hard on gravel, 2 bar and below was fine.