Sorry, engineer here!
The GPS speed question that keeps on getting asked ad nauseam usually only gets half the answer it deserves.
With many apologies to those who know this well, IAS is INDICATED Air Speed. If a plane climbs at constant IAS then, due to the decreasing air density, its actual air speed will increase progressively. This is good, because if the speed didn't increase then the lift would decrease.
IAS is an exceptionally useful parameter, GPS speed in itself is doubly useless because it is:- 1) not airspeed, and, 2) requires desnity information and calculation to be of any use.
But GPS height data could be useful, in this engineer's opinion.