The GPS might be more useful than you think. During my PPL training I usually had a Garmin H eTrex in the back. It was much more useful for Nav than analysing circuits though. (I did an awful lot of circuits
).
The absolute height accuracy can be well off, eg I tracked my first helicopter solo, and found I picked up and set down at -200ft agl, and no I wasn't in the Netherlands! However the error can be quite stable, which means descent rates can be quite accurate. Headings and turn rates require some assumption about the wind, but can also work out well.
Assuming the unit had enough satellites to start with, the big problem was when it lost or acquired one. That was when you got the sudden jumps. I was pretty sure of the reason because in a circuit the same jumps always occurred when I turned through the same headings.
If you analyse it in a spreadsheet, you can work out the energy change per time step, omit the "impossible" jumps, and look at the trends on either side. So with some effort, even "hiker GPS" data can be useful.
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Incidentally, in case any PPL students reading this might think GPS tracks are a good way to analyse circuits, my experience suggests not. I never learned anything useful from them. By contrast, GPS tracks were incredibly useful during Nav, not least for showing the FI you had actually gone where you said you did!