It stands to reason that and Aircraft's GPS will give the Ground Speed, horizontal speed of the aircraft. This will not always match the True Airspeed, Indicated Airspeed, or Mach on the indication due to winds aloft, tempature ect.
I have a hand held GPS reciever. This speed nearly always matches the speed on my car's speedometer, unless say I'm driving well into the 5th amendment on the highway.
My GPS reciever computes position in 3 dimentions. I get Elevation, Lat, and Lon,
But, does the speed computed by my reciever only compute horzontal speed,
If Im driving down a steep hill my car's speedometer says 100 Mph (To make the math easyer)
But because of the angle of the hill, I would be moving vertically at 60mph while horzontally at 80mph, would my GPS reciever comput the vertical 60mph and horzontal 80mph, or would it mearly show that I was only going 80mph?
(This hill would make a standard 6-8-10 right triangle)