personthingy: I don't think Valve can be bothered lifting a finger to do anything, tbh. Especially not something that would cost them money, rather than bring it in for free. :B
No... this attitude is entirely on the devs.
Also, I forgot to report it here, but there have been a few threads about this on their Steam forums. The dev has closed a couple of them, but has clarified their position in the most recent threads:
"We have not in any way abandoned the platform. We support every platform, including GOG, with all of our free in-game content updates. However, different platforms have received and will likely continue to receive varying levels of external content (DLC, avatars, themes, etc). Our GOG builds do not currently support external DLC so, unfortunately, it is not as easy as uploading the DLC onto the GOG Store.
We know we have players on GOG and understand that there is a desire for more content outside of the game so we will definitely keep you in mind if we look to expand the scope of DLC support."
personthingy: So basically they want us to shut up and be thankful that they even bother to release updates. Calling bs on the doesn't support dlc claim, there are plenty of multi platform games here that have dlc. So what makes the gog version so different from every other platform that dlc is impossible? It just sounds like they are relegating us to the low priority list and focusing on the "big platfroms", just like so many other publishers here.