I don't believe the GOM takes any account of actual driving conditions on the current trip - it seems to simply take the static state (mode, kers, maybe temp and some other factors like battery history/cell state) and applies that to the theoretical range and state of charge.
Perhaps more accurate would be to take your trip miles/kWh after at least 20 miles multiply that by 50 (as a good approx of the total kWh available at full charge and easier to do in your head - halve it and multiply by 100) and then apply the current charge % to it.
So if after 20 miles you are averaging 3.6 miles/kWh and you are at 33% charge then you've probably got 60 miles left. If you are getting 4.2miles/kWh then you've got 70 miles left.
At less than 20 miles done the miles/kWh is not meaningful for extrapolation.
BUT if your 20 miles has been from Princetown down to Plymouth and all at a steady 40mph and you're about to take it up to a steady 70 on the dual carriageway and up hills for the next 100miles and heavy rain is forecast then the calculation will be as far out as the GOM.
In other words use good sense - the future does not have to be the same as the past whether you are using GOM or some other calculation