What are you calling a GOM ? As Bjorn said, the range remaining (or % remaining) displayed in the upper left of the screen is not a GOM. That is why people try to use it as a surrogate degradation marker ... and fail, because the calculation has constants that are set (and perhaps changed) by Tesla
When it comes to figuring out battery capacity there are only really two half decent methods available to regular people: 1, relying on the odometer and Wh/mile meter; and 2, relying on the electrical meter during a charging session. Bjorn tries to correlate these two methods to be more confident of his results. Any other method or car reading is very unreliable.
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Two years in Colorado, now in NM
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
2018 Tesla Model 3 LR, Delivered 6/2018