One method is by taking the lowest component rating and calling that the GVWR. In most cases, that would be the axle or tire rating. In other cases, that would be the hub and wheel rating. Either way, the GVWR is determined at the axle. With this method, if your axle rating is 6,000 pounds per axle, and you have two axles, your GVWR would be 12,000 pounds.
This is what I was referring to.