The GVW won't change if you're on a hill, weight is weight, but the torque required to move the load will obviously be different. This is a paper vs real world difference, as you should be able to move the load up a hill as well as on flat ground, right ? What seems to be going on is that it's so competitive among truck makers these days that they're at the limits of the components when they're spec'd, which is why an auto might work fine on level ground but not have the load handling ability to start a max load on a hill.
Makers are at least getting smarter when they defuel the autos as they've had high failure rates in the past.