Here are the capacities that GM publishes for their Chevy/GMC 3500 series pickups. The numbers vary slightly depending on cab style, 2WD or 4WD, and bed length. The whole table is quite large; I'm going to show one specific comparison: Crew Cab, Diesel, 2WD, Long Bed.
Payload: 4327 SRW, 5548 DRW
Payload includes the pin weight, plus everything in the pickup: driver, passenger, fuel, hitch, "stuff" in the cab and bed. Following rhagfo's example, the pin weight is probably somewhere between 3600# (20%) and 4500# (25%). If "everything else" is 750#, the actual payload weight is 4350#, slightly over GM's listed capacity if the pin is 3600#, and quite a bit over if the pin is as much as 4500#.
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Gross Combined Weight Rating: 20500 SRW, 24500 DRW
GCWR is the total weight of the truck and trailer, together. GM Curb weight for the SRW version of our example truck is about 7300#, the DRW version is 7600#. With driver, passenger, etc. weighing in at 750#, we have a gross weight for the truck of 8050# (SRW) and 8350# (DRW).
The difference between GCWR and the truck's curb weight is the amount of weight the trailer can be, and not exceed the manufacturer's specifications.
Max Trailer Weight SRW = 20500 - 8050 = 12,450#
Max Trailer Weight DRW = 24500 - 8350 = 16,150#
