[AZKen]

Almost any of the harnesses sold will work with a GMC. I am speaking of harnesses that come with a prewired fuse box and wires that need trimmed and terminated. These are the ones most folks use. I am not speaking of some sort of pre branched, pre loomed, special factory original reproduction designed kit. Made for a purest.
You don't need to buy separate engine harnesses, cab harnesses and such. It's all in one kit like EZ-wire, Painless, American Autowire and all the rest.

If you run into a connector you need that is GMC specific, some of those are available.
Be careful with, and salvage, EVERYTHING on the old harness as you remove. Save it as intact as possible. You should switch to a ballast resistor instead of resistance wire during rewire, in my opinion. Easier. The wiring for this era is relatively simple.