We also have some exciting news for Mac and Linux users: on 15th of March Cossacks 3 will finally become available on MacOS and Linux platforms! Venture forth onto the battlefields of the 17th-18th century Europe and enjoy the fully polished Cossacks experience with your friends, regardless of what platform you're using - the game supports crossplatform play and allows you to play against players across all platforms.
As a bonus, every Mac & Linux user that purchases and logins into the game during the first two week after the release will receive an Early Bonus package, identical to the one given out during the Windows release.

Source: http://www.cossacks3.com/forum/index.php?threads/the-steam-workshop-is-now-enabled-mac-linux-release-dates-announced-new-patch-notes.29158/

Update (expected to be on Friday)

I see that the Linux version is finally on Steam. Are there any plans to bring it to GOG ?

I guess so. However, the Linux version is still in public Beta.

The Linux version was out on Steam for quite some time,
But still no Steam gog version?

I bought the game accidently thinking there is the Linux version on GOG... oh man, I hate this. We have it on Steam but not on GOG, AGAIN!!!

Unfortunately, I think the Linux version is still in Beta, that's why GOG doesn't have it yet.

What's with the hold up?

Add Cossacks 3 GOG version for Linux please.

In early 2019, I crave for the Linux release.

So where is the linux version?

GOG your working very hard to sell games -not really, why are there so many of the same games on steam with linux support and not here????? are you listening :(

Got this game on Steam with Linux support. Runs great on my Arch. Would love to buy DRM-Free version of this, but NO LINUX SUPPORT? Cmon GOG! You can do better than this!

Add Linux support please!