GIDs are the Leaf's internal accounting for energy, 1 GID is exactly 80Wh of reported capacity. There's really no need to know this outside of specific engineering contexts and kWh are a much more logical unit to use.
Hx is one of multiple coefficients the battery uses internally to characterize its polarization (internal resistance) characteristic. The polarization characteristic tells you how much the battery voltage decreases or increases as a function of (sustained) current draw. Batteries typically have a fast and slow response; when you suddenly draw a large current it will first decrease in voltage quickly, then continue decreasing more slowly. This characteristic is usually algebraically represented as:
dV = A(Be^(-ct) - De^(-et))
Coefficients A, B, c, D and e are tracked by the BMS. Hx is equal to 1/A.
It's hypothesized (but nobody has done the work yet) that B and D are missing from the BMS's internal resistance calculations on the first-gen Leaf, and have been added in in later generations. This would explain why it tracks SOH so well on gen1 batteries, and so poorly on later models. c and e are likely hard-coded constants.