As far as I know, the GCWR for a vehicle refers to the drivetrain only. Identical trucks can have different GCWR based on the rear end ratio alone.
For example, Fiat (Ram?) lists GCWR for a 4x4 long box extended cab ranging from 19,900 to 39,100. Those trucks all have the same axle ratings, the same brakes, the same tires. The difference is engine, trans, axle ratio, and base weight.
The highest capacity setup is rated to pull a 30,320 lb trailer. IMO, that's insane. But they say you can do it. I suppose, if you had 12,000 lb axles and could get 5,500 lbs on the tongue, you could get almost there.
Of course, all other rules still apply. You will need a CDL. You will likely need a DOT number. If you cross state lines, you need IRP, UCR, IFTA, and all the same BS that a "real" truck needs.
It is funny though. We (tons of people) move 55,000 gross with old International 4900s with the old mechanical DT466 that makes all of 220 hp. The new 6.7 Cummins is around 425 hp. It's plenty hilly here, and 50,000 lbs will have the old IH crawling up a steep grade at 20 MPH. But it'll do it. Yet theses pickup guys need tuners and twin turbo setups to pull a 3000 lb fishing boat.