Man! I havent thought about an ol Gentleman Jim in YEARS!. It made me smile when I seen that. The ones I remember where Black & Gold. They were all sharp trucks back then. Long wheel base... well they were "long wheel base" then... now I think they are "long bed". It had Gentleman Jim right on the rear quarter pannel.
To answer your question, I dont have a clue what it would be worth but I would suspect its worth a little more than a regualr truck of its time. I used to have a catalog sent to me every month that was specificly for that body style truck. It had all kinds of parts, seats, origanal dash & after market dash etc. Doors, moldings everything. I dont have any left lying around but maybe someone will know the company I am talking about. If we could get you to that site & get you a catalog, you could get anything you need right down to clips & original screws to put that thing back right. Good luck & if I think of the name of the company, I will re post.