I have owned Both GMC's and Chevy's of the same model
They are Identical trucks in the mechanical dept with a few minor differences here and there but all parts interchange.
The differences I have seen over the years is of course "Badging" "Grill designs" "Head light configuration"
Some of the trim packages will have different "Mix and Match" goodies.
The notion that the GMC was heavier duty is an old wives tale that probably got started back in the Dark ages.
I do remember a 91 GMC Burb that I owned a while back. This truck had the same outside electric side mirrors as the 92 (New body style) trucks. The mirrors were
mounted to a little bracket instead of to the door.
I have seen this on a few GMC Burbs but not on a chevy Burb
That truck had a few other transistion model items that I have never seen on a chevy.
Its all the same stuff ya know.
If you find some goodies on a GMC that you like you can retrofit the stuff to your Chevy.
A friend of mine cracked the grill on his GMC Burb and found a lovely CHROME chevy grill cheap YUP bolts right on. (95 truck)
I have owned Both GMC and Chevy Pickups and Burbs. In the dark they all look the same, HAAAAAAAAA
The front control arms are "Forged steel" not cast iron (sorry) but iron would not live in that spot.
These items were changed during production runs and done so to save on costs.
There may have been an engineering reason too but I suspect cost was the chief reason.
The Barn door/Tail gate thing is strictly an option choice in the list of a huge number of options.
We had 2 identical Burbs right down to the color (6.5 TD)
Same 8600 GVW and all. One had barn doors one had the gate and flip window. That was the only difference.
I have sat down with the option book and ordered a new Burb before and speced out exactly what I wanted.
There is a huge amount of little goodies that area available.
Some options require you to order a particular package to get.
Other stuff does not require a package.
Most rigs are ordered by the dealers for stock and they simply order a common "RPO" group (one that sells well)
Common engine and tranny combinations,trim packages, gear ratios, Paint colors, etc.
Other than the badging and grill designs most stuff is available across the lines. (at least in recent years)
MGW