dng unloadable
I have exactly, I mean precisely, the same issue specified in the "closed" issue from this thread ( #2083 (closed) ). I am using the latest stable versions of both gimp and either darktable and rawtherapee (tested separately) with the same failed result and error message specified in the closed thread. I may have missed it but I could not find a reason for closing the issue. Typically it's because an issue is either resolved or because it is something the developer has absolutely no control over. In this case, the latter is unlikely given how compatibility over this feature is featured and advertised by both parties.
One guy in the closed issue thread said it best when he mentioned how this is kind of a big deal because .dng raw really has become an industry standard.
Not something you want to sneeze at if you want to be taken at all seriously.
P.S. I just discovered that Krita (mostly known as a drawing app) supports dng out of the box (0 plugins). I saw they're actually #1 on some rankings this year for open-source image editing.