IIRC it was here until it was removed because it became Epic Store Exclusive. Then it returned to Steam store but not here. Is there a reason?

A lot of things happened since then.

Long story short, the devs had a lot of issues with the publisher, they went to court, the publisher seemingly cracked another version of the game and uploaded to Steam since the devs didn't want to give their Steam build, the devs issued a DMCA takedown on this "new" version of the game, the game vanished for a while and now all of sudden it returned without warning.

And I don't think this story is close to an end, so it'll probably take some more time until/if we see the game here.

That city has sunk, and even Cthulhu isn’t bringing it back.

There is a very small hope.
I own the steam version and there is a GOG Galaxy folder in the binaries folder.
we will have to wait and see I guess.

But Steam sells the version published by Nacon, which not only lacks DLCs (if latest game reviews there are true) but also sells against developer's best wishes, the Frogwares. So that version is broken and it must not affect GOG release in any way other than court disputes between the two parties.
If we ever get the game here it would be published by Frogwares directly with all the DLCs.

So I think that Frogwares can't publish the game here because the court decision is not ready yet, or maybe GOG doesn't want to publish anything with ongoing court process, or maybe even Frogwares have not decided whether or not to release their game on GOG.

Now we have a fight vs. a giant and a giant rarely is losing in todays world owned by the one with might and bucks.

Surely funny reading all the comments on the Steam page.. this game seems to be one of the few games broken on Steam... and while GoG users are experiencing this more common... a spoiled Steam-user can become pretty upset it seems.
https://store.steampowered.com/app/750130/The_Sinking_City/

Sure, to Valve/Steam taking down games is a unfortunate event they don`t enjoy to see, especially they want to make users believe "you always own your game". Truth is... it may not necessarely be true and no game is owned better or with a better license than those found on GoG.

I was following this for a while. Frogwares does have their official version on another site but not here or Steam. They have it on Games Planet.
Links here:

The Sinking City
The Sinking City - Deluxe Edition

Check their site blog here so you get all the info on what happened on their end and why the links to buy their game is where it is.

Wow, thank you everyone. I didn't think it would be this messy. The GamesPlanet version sounds interesting but I still wish it would come to GOG later.

Perhaps this is a stupid question, but are those versions on Gamesplanet DRM-free? Is there any client or anything required to download, any restrictions on them, etc?

It might do - I suspect that GoG is taking the sensible approach of sitting back and waiting to see how the legal cases pan out.

I remember reading it has some sort of DRM, just don't remember which. And I think that's the version Nacon supposedly cracked to publish on Steam.

The Gamesplanet version is definitely DRM-free. I bought it myself some time ago. You get an offline installer which can be downloaded directly on their website, no client necessary.

This is a sleeper hit. When it comes to GOG, I'll be purchasing it again to show support for Frogwares.

Thanks for answering. I was resting for a while before I saw this again. I recall them saying it was DRM free but I haven't messed with it yet.

When it comes to games needing all these different sites to use to download for whatever reasons they may have.. it is a bit much to keep up with. I have pretty much been using GOG for games I want DRM free for years. Any games I didn't mind or care as much having some sort of DRM I would get on Steam until they appear on GOG.. which if the game is worth it, I'll get it on GOG and be done with the Steam version when possible.

afaik it's as simple as going to the Frogwares site and registering, then buying the game and getting a downloadable exe file. Not 100% though.