On Steam there is a 'Premium edition' that includes "Original Soundtrack (41 tracks), 8 audio books (160 minutes total running time) and a wonderful game comic by Olga Andriyenko". Are these included in the GoG version or, are they available/going to be available on GoG?

The current GOG extras are just wallpapers, avatars and artworks. At the point when TNotR was released, GOG didn't sell (IIRC) multiple editions of one game, so Daedalic went with the optimal price solution (quote). This seems to be changing nowadays, so maybe there's a chance of the Premium edition appearing here.

Thanks. That's a shame, I'd like to get the premium edition DRM free but, I'm not fussed about waiting (have plenty of great games to play in the meantime) so, I'll wait and hope it's available on GoG in the future.

Ya, I was disappointed the complete version is only on Steam as well.

I just become aware that a premium edition exists and I created a wish on GOG to include it. The name of the wish is "The night of the rabbit : Premium edition".
I hope this will come true.

Because nothing is impossible.

The Steam version is DRM free, too.
However, I just noticed, that the older versions do have DRM on Steam. Only the updated version is free of DRM. This might be relevant, as they always manage to make things worse with updates. While adding features, they also break things quite often. Also this one is another victim of graphics reduced to lossy WEBP for no good reason on the recent Steam version.
You might even consider it a good thing that they don't update their GOG versions anymore, it seems.

And for some reason, the music is in mono in this game in all versions while the separate soundtrack is stereo. Same thing with Deponia Doomsday. I don't understand why they downgrade their games for no reason all the time.