My GoM is reasonably accurate for low to medium speed highway (not freeway) driving. I'm attaching an example of that here.
I used exactly 59kWh for that trip. My capacity at full is 81kWh. So this was 59/81 = 73% of full charge.
So I traveled 184.2 miles using 73%, at a decent average speed ... and what you can't see is that this average was due to a very tight distribution of speeds, between 30 mph and never over 62 mph.
So my range was 184.2 miles / 0.73 = 252 miles!
And notice that the measured efficiency was apparently 32.9 kWh / 100 miles, implying a range of 81 kWh / 0.329 kWh/mile = 246 miles.
In stark contrast, the next segment on that same day, on I-5, was at an average speed of 64 mph, with a lot of miles at 75 mph and a measured efficiency of 39.9 kWh / 100 miles. I used 61.2 kWh for that segment, implying a range of 149.5 miles / (61.2/81) = 198 miles. Which is very close to the ~200 miles implied by the consumption estimate (which btw is always off a bit more when there is high variance in speed along a route).
The GoM was always somewhat wrong. The first trip above was after a high speed high consumption day, so the GoM somewhat underestimated my range. The next day the GoM way overestimated my range by a lot because the day before it was so efficient and now I was hammering it.
I've made it a habit to watch the GoM decrease and match that up with the number of miles driven, to predict the trajectory.
For example: the first 50 miles of driving cost me 60 GoM miles; so if I have another 100 miles to go, I need to subtract 20 miles from the GoM to see how much I have left when I get to the charger.