If you are asking what the truck is, it doesn't look like a factory product, but rather something someone scabbed together or which possibly was put together by a jobber.
The grill says GMC and its a GMC dash, but your pictures don't show anything else I've identified that would be only GMC.
I don't understand the urgency to know what it is. If I owned it I would use it as a yard-bug for moving things around. The sign on the door suggests it may have been used that way when modified into its present configuration.
Does it have a serial number? That would at least help identify its origins. Something like that you're going to be measuring and researching when you need parts because you've no way of knowing what is original and what isn't.
