GPS is irrelevant to this thread.
GPS position is irrelevant to this thread. Even if the speed of the aircraft relative to the Earth was relevant, the 737 does not use GPS to display ground speed. It uses the calculated FMC position, which is derived from the IRS position, the triangulated DME position, and the GPS position via a complex formula. At times, the GPS input to the FMC may be ignored altogether. And in fact, GPS is not even required for non-EDTO (Extended over water) 737 operations.