GMC 3500 capability

I am not sure I understand your question but I will spread it out like this.
Payload is how much added weight the vehicle is rated for. That is the weight carried on the truck itself.
GVWR: Gross Vehicle Weight Rating. That is the maxium weight the truck is rated to weigh, truck,load, you, etc. My F150 is 7106, F250 is 8800, Dodge 3500 is 11,000 and change, 4500 Chevy is 17500.
CGVWR or GCW is combined gross vehicle weight rating: Total truck trailer and load weight. To me that is what you are worried about to be legal. You might have to look at the owers manual to find that on some smaller trucks.
The F250 ( 6.0 auto) with a trailer grossing 20,000 has a hard pull in the hills. If asked to do that on a regular basis I am afraid it would have a stroke.
The Dodge 3500 ( Cummins manual ) grossing 15,000 on the bed is happy locally. With a trailer and more weight I am comfortable its just not fast.
The 4500 has a GCW in the door for 30,000 lbs. It is not a hot rod but it has the transmission, brakes and tires to do the job. All day any day anywhere.
A lot can be done with a smaller truck if driven correctly but it is always better to have some extra.
Just my understanding and opion.
A freind of mine bought an F450 1 or 2 years ago. It is impressive.

 

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I put 200K miles on a 2014 Duramax. Good driving truck, strong engine, decent fuel economy. But once you hit 150K miles, get ready for problems. The DEF system is problematic, and the catalyst went out at 175K miles to the tune of $3400.00. Both of the ones my employer has needed injectors and one needed a turbo before they hit 150K miles. Both trucks have been well maintained and not abused, it just seems they are not built to work that hard every day.

 

You might be able to pull your drunk cousins chevy suburban out of the mud.