Mark, I got the exact Gem Line replacement gasket from Modern Parts in Parma a few years ago. It wasn't cheap, but with the old gasket I had major frost building up within a week of defrosting, so the new one was well worth it. Had I known the results would be so fantastic, I wouldn't have waited so long to install it.
You should give Modern a try. Last I heard, relatives have kept the business going since Larry's passing a year or two ago.
Ralph