Yes, GVWR minus curb weight. The best way to get your curb weight is to load the vehicle with a normal load (full tank of fuel, normal passenger complement, etc) and get the vehicle weighed. I don't believe that CAT scales have a problem with pickup trucks, although I don't know what they'd say about a passenger car.

And, yes, GVWR means "Gross Vehicle Weight Rating". It's based on what the springs, and more importantly the brakes, can handle.