I emailed them about this a short while ago and got a reply today:
Hello, unfortunately, we have a big problem with moving version up on GOG.com, since the engine GameMaker Studio 1.4 is deprecated now and BLACKHOLE is still not working with GameMaker Studio 2, we have big problems to patch our game.
But we are still working on it. It's a long process where Engine developer needs to adjust a things for us, which is not easy by their words.
I am really sorry for this inconvience, but it's not our intention.
GOG.com have all the DLC though. And the version 1.9 on Steam is basically the same as GOG.com 1.7 ... there are few known issues and bugs though. But we are using different version scheme between Steam and GOG, so that's why it's little confusing.
Thank you for understanding,
Make of that what you will.