Maighstir: The publisher most likely realise that GOG is a fairly small player among download stores, and Linux is pretty much an infinitesimal player among operating systems used by gamers, thus there may be some gain in releasing for Linux (on the largest distributor: Steam), and on GOG (for the most common OS: Windows), but making a Linux release for GOG means they're not going to regain the cost.

sunshinecorp: What cost? If there's a linux binary already, it costs nothing but a bit of extra time to keep it up to date on GOG.

Maki: I have Cities in Motion on Steam, where there is a Linux version available.
Where is GOG's Linux version of this game?

I have noticed there are more games which have official Linux versions made available by their developers, which are then released on Valve's Steam or other places (Humble, Itch, etc), but not on GOG.

Why this disparity? What is stopping these developers from releasing their Linux versions alongside the Windows/Mac versions?

I really would like to see a clear answer to this somewhere, as I have not been able to figure this out (My apologies if the answer IS somewhere and I have not seen it yet; I must have managed to run straight past it while trying to find this answer in the past hours. (Basically since receiving the "Up to 80% off Stardock and Paradox goodness" email, which contained the aforementioned game - only for Windows and Mac.)

Maighstir: Yeah, time -> salary -> cost.
And more time if the Steam release uses some form of DRM.

If the port's outsourced, the cost for each ported release is likely higher.