Cool project!
On the GPS G-units, they come directly from the NMEA datastream from the GPS receiver, so it depends on which receiver you use how the G-force is being calculated.
The problem with the 'strange' readings is, that the jetcat ECU only shows min/max/average values, you always have some strange spikes, and that's what is reported as min/max (I dont believe there is any intelligent filtering inside)
Averages are always quite good, so I'd vote for showing the G-force data from the ECU. At least on my rookie it always looks like valid data.
Also, maybe one button to toggle on/off the display? (maybe I didn't read correct, and it is in there :P )
Hans