Everything I have heard about the Atlas O Trainman Gon says it was based on a Pennsy Prototype.
The original car was designed and released in the 80s by Atlas. The scuttlebutt is that a guy named Ted Stepek did the design and layout for the 4 cars Atlas did in that era. The gon, a 40 ft sliding dr boxcar, a 40 ft plugdoor boxcar and a modern caboose. The Gon and the 40 ft sliding door car are supposed to be based on Pennsy cars. The Plugdoor box is supposedly imaginary and the caboose has a modern prototype. Ted Stepek was a train buddy with John Armstrong - a bit of trivia - and if you have John's track planning books, a layout in one of them is called the Schulkill Division and was a plan that Ted built.
If you model the 40s, you need the the "circle keystone" paint schems on Pennsy cars. The shadow keystone started to appear about 1955. the final scheme was just the initials and number was in the early 60s I think.
The new Atlas O car appears to have the identical superstructure to the Atlas car from the 80 but has a modernized underframe and trucks. These cars work out quite well with a little weathering.
If you have shopping time you can often find these old gons at train shows for 10 or so but you have repaint and replace trucks and couplers.
