Actually it depends on three things. First is the frame, second is the suspension and third is the powertrain.
The frame has to be strong enough to hold all the powertrain components and suspension when a load is applied, and strong enough to and pull the weight when power is applied. The suspension has to be strong enough to support the load and (GVCW). The powertrain has to have the power, torque, axle strength along with gear ratio to pull the load along a desired path. With the same truck but gas vs. diesel, the truck with the diesel engine and heavy duty powertrain has a higher GVCWR (per owners manual and GM web site).
I have worked on my friends 2005 Chevrolet 3500 SRW truck and the frame on it looks to both of us as the same. I could not see any difference except for the beefier suspension with the helper springs and heavier rear axle.
As for the engineer he said that the frames on the GM trucks can safely take a heavier load then the manual states. However he does not recommend going over the GVWR/GVCWR due to liability issues.
As for me, I am still within my GVCWR and do not ever plan to go over it!!! Also, the up grades I have done just makes the platform more stable.