Gas, Diesel, Gears and Towing

Help me to understand the differences. Why can three trucks that are identical other than engines and transmissions (understanding that a diesel truck will have a higher GVWR and a lower payload because the engine weighs more than a gas engine), have towing specs that are so dramatically different. For instance, Truck A has a Gas engine and 3.73 gears. Truck B has the same gas engine and 4.10 gears and Truck C has a diesel engine and 3.55 gears. Why does Truck B have such a higher GCWR than Truck A, while Truck C has a much higher GCWR than both Truck A and Truck B but has a 3.55 gear ratio? Is it all about torque and HP? Why do manufacturers set such different GCWR allowances? Is it about wear and tear and over working engines and transmissions on trucks with gas engines that provide less torque and HP so things can break easier? Or is the Diesel truck not identical in other ways like a diesel engine will have an exhaust brake or brakes that are more stout as a standard feature so more weight can be stopped safely? I know the easy answer is "Don't worry about it. Just tow with a Diesel Dually" but I would really like to learn and understand. Thanks in advance!