I don't know (yet?) how and where these cars were made. All I see is that when you search on the Net or in books about the Advance-Design series, you find almost exclusively information about the Chevy trucks.
Even in IMCDb, you can find a least one GMC pickup listed as a Chevrolet one


I just say we cannot know. And as there is no better view of that truck in the movie, we have no way to say more for now⁽¹⁾.

Either we accept this and find a way to note it in the id. Or we draw lots for the make and the model and take responsibility to say possible craps to the others contributors and to the visitors of IMCDb.
But if we do it for just one car, why not for any one we cannot identify? And why not for some others, depending on our mood and/or tiredness? What kind of credibility would we have, then, for the rest of our work?

Personally, as a (modest) member of it, I would prefer IMCDb crew to be honest
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(1): I admit it's hoping a lot, but maybe some day one of the contributors/visitors will read an article/see a documentary about the cars and the trucks of that movie and then will be able to "de-unidentify" this truck?
Let's be optimistic