How the GPS calculates ground speed. How it is different from ground speed calculated by DME. Whether these two are same are there is a difference between two.

DME is transponder based, measuring distance from a fixed location. GPS measures rate of change in location based on a coordinate sysytem.

To add, Gps is more accurate due to slant angle, Gps is determined by points over ground, and DME is determined by its points over ground and the altitude difference to the station. DME could be considered more accurate as it relates more to your Flightpath.