The Gid and kWh numbers have a clearly relation to the battery voltage and Ahr. Are you skeptical of those too ?
Gids have no units other than the unit named in GaryGid's honor. Who knows what the Nissan internal name is? I'm not particularly skeptical of gids other than to look at them as a unit-less value that we can use as a proxy for a finer grained state of charge value where the max also decreases as the battery degrades. To start assigning a constant of 77.5 watt-hours/mile then multiply to result in kWh I'm somewhat skeptical of.
For all we know, Nissan's internal documentation could say something like, this value is not valid outside of a certain numeric range or, it's only valid between certain temps or after a ___ discharge cycles down to ____ and then to full. Or, between x and y gids, you should use z constant. Between and and b gids, you should use c constant. Or, there are known quirks below d gids or above e gids.
The Ahr figure could have similar caveats about pre-conditions.
I'm not particularly skeptical about cell or pack voltages based on known specs of the pack and cells. I haven't followed all the CAN related work, but I'd imagine work like at http://www.mynissanleaf.com/viewtopic.php?f=8&t=17470
further confirms the voltage values being rendered at the cell level. I don't know how accurate those sensors are, but it doesn't really matter that much for many/most Leaf Spy users.
It is useful to compare notes about figures like SOH, Hx, AHr and # of gids on a full charge to help determine how close someone is to losing a/another bar.