I don't think that is quite correct. Even though in theory they can measure high speeds (which would be limited by their ability to keep the shifting signals locked - a processing limitation rather than a speed of light limitation), I believe that civilian GPS units are required under ITAR not to report speeds in excess of 1000 km/hr to prevent them being used for weapons system applications.
Only for GPS units capable of functioning above 18,000 meters altitude. The limit is 515 meters/second (1001 knots). And those limits apply only to GPS units built for export. Otherwise you need a State Department export license to export the GPS. You can get a permit for darned near anything these days.
Thanks for the clarification.