GMC as well as most larger trucks used a positive ground because lights, especially on the trailer, loose ground a lot quicker due to corrosion of dissimilar metals with negative ground. In fact Freightliner used positive ground until the 70's when drivers got electronic gadgets such as cb radios and demanded negative ground. Most trailers today are built with 2 wires going to each light instead of grounding to the chassis. GMC was built as a big truck while chev didn't get bigger than 2 ton until the task force series. Trucks did have less grounding problems with positive ground. I was driving them and fixing them in those days. I don't want to go into a lengthy discussion, but if you read about corrosion between two unlike metals, it all depends on which way the current if flowing.
