Is it going to be publisehd in GOG? If so, when?
Thanks.

The treasures of the past DLC was offered by GOG.com only as a pre-order DLC. Therefore the DLC will not be shown. You can't buy the DLC separately.

Might be something to do with Warhorse, though, and their agreement with Steam.

I know, from where you stand it does not matter, but it probably wouldn't hurt to e-mail both Warhorse and GOG asking for adding the pre-order bonus to their catalogue. Squeaky wheel and all that.

May I ask what is your another regret for buying here versus on Steam?

Personally, I prefer non-DRM versions if price is not substantially lower. Also GOG doesn't force me to apply the latest patch unlike Steam. Also, for pre-orders, the DLC was for free. I see it now for 4,99€ on Steam.

As for the DLC itself, it's only the warhorse armor set, which is not a big deal in the game, in my humble opinion. I would see a value in it only if you really like how it looks like. I personally ended up using only the helmet for the stats and it looked bad in cutscenes (the helmet itself was not visible in cutscenes, but the red&white circle was, which looked very weird; but maybe they fixed it already, I don't wear the helmet in my second playthrough).

The late release date.

I prefer non-drm versions too, but sometimes, like this game, when the hype is very high, some seller decisions may push people to other places that gives more reasonable options. In this case, i was very hyped to play this game on release date (Didn´t see the GOG release date before buying), and could only play it many days later... when my willingness to play was much lower.

About the DLC, this particular one may not be so impressive, but people may want to have the full game experience. As @Lukaszmik said, the best way is to contact the developers and GOG to see about this DLC. That´s what i just did. Let´s see.

Fair enough. It sucks that the release date wasn't more emphasised on the GOG site. I decided to buy the game after the Steam release, after I enjoyed watching the gameplay on twitch. Since I generally prefer GOG over Steam, I decided to wait a bit further for the GOG release, to start my playthrough after more bugfixes (I had hoped for more fixes by then, but at least some stuff was fixed).

The DLC has been released here at GOG. Thanks all.

Awesome, glad you were able to get the goodies.