There is a Mac version; it's on Steam. There is no Linux version at all.

You do know developers don't handle any of that? They just upload the new version and GOG does the Galaxy/offline installer stuff.

It looks fun at least

From several conversation with developers this can not be further from truth.
1) GOG needs a lot of time to get most GNU/Linux versions (updates incl.) on the Offline Installer -
and there is no GOG Galaxy Client for GNU/Linux (not even planned).
2) This time is explained by quality assurance - good joke. Several games did not even start.
3) I looked for changelogs - und guess what - devs put it to GOG - and - missing from offline installer.
Additionally GOG Offline Installer does not allow Cut&Paste - so all Changelogs in the forum comes
from Steam.
4) Even when developers put new versions to GOG - they may not reach the offline installer (not kidding).
As I am no game dev, I don't know what info devs get from GOG or if you need to have a responsive
person to address in some cases.

So summarized I am still believing that one person does the entire GOG website.
And each thing must be done by hand ...
GNU/Linux is totally neglected - and DRM free is only for SiPl mode.
Yes, the former GOG customers are not pleased - and those seeing the love and care GOG had
for their strong points are not happy either ... (fresh DOSBox games incl. manuals written by GOG).
Currently they would not update the used DOSBox (which can go crazy with current GNU/Linux distros)
and in most cases the got Windows only.

By the way, I got message that a GOG member will look at my support call ... but nothing happened.
This is also normal since I am a GOG customer (2 years) ... so in this time the situation got worse
and worse ...
I hope that sometimes this changes - but GOG is on the way to get a bad Steam ... this hurts.

Maybe I must make my point clearer:
If the Dev only puts it to GOG and does not make sure it will reach Offline Installer, than you may wait
indefinitely to get the update and blame the Dev for treating GOG customers as second class while
in THIS CASE it was GOG not pushing it to Offline Installer.
And no, this is not a rare case - I had several times contacted Devs who said to have pushed it to GOG,
so similar to some tickets getting ignored it is the same with updates.
And we pay GOG in first place, so GOG should take care to get updates - even trigger Devs to send them -
but it seems GOG is less interested to make their customers happy than the Devs.
So, now you should see that is not a matter of GOG doing everything but that Devs do the main thing and
should control if the things were done correctly.

And in this case GOG should know that there are non-Windows customers who bought sequels of that series
and should feel obliged to get info if this will come later or if there are reasons they will not reach GOG.
A mac version exists on Steam - and as these Devs have experience how to make Mac and Linux version,
a polite question to get an answer for their customers would be in order.
And this should be part of the announcement of any game. I don't think this is too much to ask for -
and it will help Devs to concentrate on their main work instead of answering the same questions again and again.

So if you can do it yourself you have less to do than if you put it in a form GOG wants it (e.g. DRM free) and
than make sure every thing works as expected. And I have heard that Devs prefer to do it themselves (which
is the case for Steam website and other services, AFAIK - and no, I would not buy anything from Steam - just
saying that there are some points which seem to be much smoother for Devs - which means less money:
this was the point of Ancient-Red-Dragon which you contradicted and my point to give the picture I had through
several Devs ... so while your points are valid, your implication may suffer due to other factors I tried to give).

You still don't get it. Devs DO NOT have anything to do with offline installers. That's 100% GOG.

You still don't get it - GOG is GOG people and not GOG as the platform - so after putting your game
to them does not mean that GOG makes sure it lands on the Offline Installer - so the Devs must make sure
that it lands.
As Devs can not put it directly to Offline Installers (why???), you rely on them - but this goes wrong
much to often.
That is the point here.

And as customers of GOG you should get an info from GOG like "Unfortunately there are no plans
(as of yet) to port KRV and IM to GNU/Linux." as I got from Ironhide Game Studio on 20.12.2021.
As many bought it from GOG and would expect the newer titles to appear soon ... but this seems
extremely unlikely ... and for people who know something about port, layers and emulators, this is a big thing.
So what I say is that GOG does not care - it's the masses (gamers who don't care - playing a game and forget) -
not something like "feeling at home". And that is a business fault which will get clearer in future.
GOG is not number one ... so that's it.

Yes, that's what it means. Offline installers are 100% on GOG. No. No. That's nothing to do with GOG (they do not make or port games), 100% to do with devs. You really, really do not understand how any of this works.

Nice, so you think that after Devs put their code to GOG everything will work perfectly.
And if you go to a shop, you don't expect to get info about other versions not currently in store which will soon come?
Maybe you should apply for a job at GOG - I think your opinion would be well suited to the GOG support.

As an IT professional I was really shocked to see that GOG seems to be a merchandising platform - and
things like "gamers should feel at home" is just a joke.
I saw that gamers provide the essential information and that GOG support is the required (by law) refund
if the product does not work.

The Devs try to care - sending Updates and Changelogs - and GOG just let some of them falling through the cracks.
Really nice. This is nothing I want to pay for ... if you do so it's ok for me - but this is not normal.
As GOG make the product page and put things to the offline installer incl. changelog, the are responsible for
that content. Same for giving the OS it supports - and supporting OSs out of support is also not normal.
And putting "stellar support" on their product pages and not responding in more than two weeks ... yes,
it may be perfect for you as a customer, right?

The Developers may take time - and are busy - GOG is paid for support - and GOG advertised its stellar support.
And no, this is nothing I can confirm to be true.

I know. Because it's not. GOG does not make the product page, the developers provide all the information. GOG just posts what they're given. Developers make the screenshots, the trailers, the game description, the system requirements, the changelogs, and the games. If they don't provide changelogs then GOG can't post them. None of this is "my opinion." There is no opinion, only the way things are. Wanting things to be different doesn't magically make them different. I sure hope not. At last you're starting to get it. GOG sells games and takes care of tech issues like running the site and providing downloads (including offline installers). That's mostly it. They're a store, not your friend. I can't imagine why you're "shocked".

The original Kingdom Rush was recently updated with the 10th anniversary version. It's currently up to date.

There is something new to report about KRV - here main part of conversation with support (Ironhide Game Studio):
~~
JMB: # 18.12.2021 20:07 CET
I was happy to see that your new game "Kingdom Rush IV - Vengeance"
is now available on GOG:
* https://www.gog.com/news/release_kingdom_rush_vengeance
but unfortunately no longer for GNU/Linux (and not for macOS, but
the latter may be problematic in the current situation).
Will the GNU/Linux port follow soon - or are there technical problems
with the porting?
As your games work flawlessly on GNU/Linux, I was really astonished that
a Linux port is not present on day one - and not amused that GOG had no
word that your series being available in 3 platforms are now only
available for Windows.
I am not the only one who would have bought your new game directly -
but I need a GNU/Linux port.
Is this a new trend as Iron Marines was available on two platforms -
but not on GNU/Linux.
In the former mail I asked about the used game engine ... it may be
'Löve/Love2d with SDL2' - but would be happy to update my overview
on your games on GOG forum:
* https://www.gog.com/forum/kingdom_rush_series/kingdom_rush_saga_technical_overview_gnulinux_centric
with some information form you.
So anything you can explain I will happily share with other gamers
on GOG forum.
And your games are really great and deserve a native port for GNU/Linux.
#
IRONHIDE GAME STUDIO: # 20.12.2021 17:40 CET
Unfortunately there are no plans (as of yet) to port KRV and IM to
GNU/Linux.
Also yes, you're correct that our game engine is Love2d.
Thank you for your support.
#
JMB: # 20.12.2021 17:56 CET
Could you give some inside why the GNU/Linux porting was stopped - Love2D is
written in C++, so it should be portable a typically just a different
compile target - or is this picture much to simplistic to give reality?
And from my point of view the older games were ported well, so I am just
curious why there is no longer support for GNU/Linux.
Would be happy to share you answer with other gamers on GOG forum.
#
IRONHIDE GAME STUDIO: # 22.12.2021 21:10 CET
So I was talking with cigumo, one of our lead programmers, and he shared
with me that KRV is not made with love2d, but with cocos2d-x version 3.x which
isn't easily ported to linux. As a small teams with lots of projects it just
isn't something that we can do at the moment, but still we appreciate
the feedback and will definitly consider it in the future.
~~
So it looks bad for the moment - but still a spark of hope.
For a change some real facts instead the usual small talk and pseudo discussion.