I put the following review on GOG which was put down by GOG:

===
I own Rainswept, which is OK and working ... so I was shocked when looking at this game:
The Demo is Windows + Mac only - so we can not test it ourselves.
Currently one can read "NVIDIA GPUs WITH CLOSED SOURCE DRIVERS ONLY" and
no one can test if AMD (or Intel if someone still use this) will work - maybe under
some special conditions.
As Nvidia never participated to Linux/Mesa development for desktop GPUs
(except in some corner cases), it is Windows only with a fat proprietary
binary driver under GNU/Linux distros with kernel-part, main X.org part
and entire Mesa part, so this has nothing to do with Linux or Mesa at
all - so no Linux card. It may work with luck, but there is no real support
(thus "tainted" was introduced - driven by Red Hat developers - to mark
 that support on GNU/Linux distros is not possible as source code
is not available in mainline so is not maintained and thus driver will
break on new kernel frequently; one may check LKML with a lot of
interesting threads about Nvidia problems) and going to Steam forum
there is much noise. So for real Linux users (not on a Windows setting
with Nvidia GPU) a demo IS necessary - but unfortunately GOG does not
care for Linux and not even answers support tickets ... Thus this heads
up. If this is solved I would have a look at it myself - but under these
conditions it would be a waste of time. So to the devs: Please make the
Linux demo available on GOG - and I am sure there are several
experienced testers on GOG - me included - to help solve the issue.
===

So I have to put this information here so those who even visit the forum
(a review is much better visible) can get this information.

I have called this putting down of my review as censorship as there is a mass
of reviews of people not owning a single GOG game (and GOG has in most
cases different versions from other platforms) or don't give any argument.
So please comment here if you think that this is a review for Linux gamers
(as a heads up) or if you consider this as not appropriate as a review at all.
Thanks in advance to all commenters below this posting - and I am happy to learn.
Currently I just got opinions that this is not OK. If so, GOG should learn form this.
And this can only happen by gamers ... and yes, due to my ticket I have not contributed
to other threads about non existent Linux support on GOG and Boycotting GOG, e.g.:
* https://www.gog.com/forum/general/does_gog_still_support_linux
* https://www.gog.com/forum/general/boycotting_gog_2021
I will put a lot of facts to such threads but wait a little for GOG support to answer.

As side note to be fair: the tone of the ticket was OK - but calling this my ticket is not correct -
my ticket from beginning of 02/2021 was not answered - after informing me that now someone
is working on the ticket and I would soon get a response there was short thereafter a message
if I want to close it - and never heard of that ticket again with subject:
"No Linux support and generally bad quality concerning Linux games", support ticket ID #993749.
So I demand an explanation in the new ticket about putting down my review: ticket ID #1039690,
and to explain what the status of MY TICKET is.

But as the supporter asked me to contact the developers, I will do so directly after completing this forum post
and mail the developers ... and as said before "Rainswept" is a real Linux game - so there is hope.
So if the situation can be improved or I would get some info while AMD and Intel would not work (even though
I could not imagine any reason - Linux on desktop is currently AMD centered) I will put it in this thread.

A review would consist of your impressions of the game/demo. What you wrote is a rant, no surprise it was removed.

it is best to keep out ideology out of gaming, and better reviews.

no wonder, why your "review" got removed, it had nothing to do with the game.

What demo?

Well, I understand the point of saying this is no real review - but this is no rant.

The game has a demo - I got it just to see that it was Windows and macOS only and only the Linux version has as Minimum Nvidia with Proprietary Driver - while Windows and AMD are given as happy with AMD.
There had been cases were a refund was denied as the point was mentioned as small line like here.
And that Nvidia with proprietary driver is barely acceptable as Linux game is technically correct (kernel is "tainted").

I asked the developer for the Linux demo - there may be some misunderstanding and it was in a hurry - so I wait for further clarification - or the Linux demo to arrive.
It was said that there is a Linux demo - as I own it I can clearly say that only the Windows and Mac version are present in the offline installer (at least for me) - and this is the only way Linux users could get it - or is there a misunderstanding?

I have seen a lot of reviews I found inacceptable as giving no argument - and yes if the info does not seem to be misleading I would have just done a forum post - as in all other cases (e.g. having a lot of bugs which would be important to give as important ingredient in a review). I don't want to harm neither the devs nor GOG - but making sure that the info is correct.

So currently no one who commented yet can see any value for someone who wants to buy this game under Linux.
If this is what gamers think I will never do such a thing again - but I would be really surprised if that is the general opinion.

I am sorry, but my support ticket was not answered till 2.5 months - so asking politely GOG support never worked for me.
And maybe what I am doing is all rant for some people - and if it is me I can change this easily.

I want that start.sh will directly start the game without changing anything, I want to get all libraries included which are necessary to play it at least with distributions said to be supported on the product page, I want to see if it is a Linux game (native port) or using layering like Wine or emulators like DOSBox, I want that a Linux version is withdrawn if the version on GOG is not playable and can not be fixed by Devs or GOG, I want all native Linux ports to be present on GOG if the game is present for any other platform, I would wish for seeing sometimes solutions provided by gamers (me included) directly within the offline installer at least at some time.
I gave examples how to make games working by adding libraries, using new AGS game engines, using freely available OSEs ... and tried GOG to provide them - and currently only developers to procide them (with at least a little bit better chance) - but this never really worked for me - and also others gave up.
So that may be the road for me, too. And it is perfectly OK with me.

And thanks for the link - if this thread is true that using Vulkan is sufficient to make the game run it will work perfectly under Linux, as neither AMD GPUs/APUs nor Intel GPUs/iGPU lack Vulkan support. So the line should read "For Nvidia GPUs the closed source driver is needed, AMD and Intel is OK" - and that driver is the default as Nouveau has the prominent reclocking problem AFAIK.
I directly got the demo - would have played it and maybe gave a real review - but the blocking as Linux version is not present while only Linux has this restriction in combination with the statement of minimum being "NVIDIA GPUS WITH CLOSED SOURCE DRIVERS ONLY" was clear for me.
So my review and this forum thread boil down to "why is the product page not technically correct" with a subnote about "why is the Linux demo not present when the demo for Windows and Mac was present" ... and this is also a rant.

And than it is true that there is no substance in my review at all ... but the GOG product page (even not having the Steam link) should be changed (and maybe the start.sh, too - see below). So sorry to bother anyone with my "rant" - which I still don't get to be a rant. But this seems to be my personal problem.
Maybe I worked to long in IT business and am accustomed to technical information only - not merchandising happyness ...

But many thanks for the solution:
./"Forgotten Fields.x86_64" -force-vulkan
so adding the "-force-vulkan" option when invoking the game from the start.sh script. We all want to edit scripts ...
I just wonder if this should be the default under Linux anyway ... sorry for ranting again - I am out.

As of Wed., 05.05.2021 (today) Linux users (Offline Installer only - no GOG Client for Linux!) have now access
to the Demo version 0.9, while Windows and macOS have 1.0 since 14.04.2021, AFAIK!

The facts:
   o Nvidia does not work with Vulkan (Graphic errors under 1920x1080 - 01*.jpg; 4k just ends game) -
      while using OpenGL it is really slow (using 4k, of cause).
   o AMD (and Intel after some Forum users) does not work with OpenGL (main screen is black -
      02*.jpg) - only with Vulkan (which is better, of cause - see 03*.jpg).
   o Unity starts with a menu asking for resolution, quality and possibility to change key - seems to be an old Unity engine
      (about 2017? - as after some point all Unity games change resolution in game).
   o Config is in "~/.config/unity3d/Frostwood\ Interactive/Forgotten\ Fields", while with former game it
      was used: "~/.config/unity3d/FrostwoodInteractive/Rainswept".

I would assume that there are changes to the Unity engine as I have never had problems with OpenGL and Unity
on my system (AMD Ryzen 5 3600 [Zen2] and Radeon RX 5700 XT [RDNA1/Navi10]) - and same it true for my friend
never experiencing problems with Vulkan and such a low performance (Intel Skylake and Nvidia GTX 1060 3GB).
So this should be fixed ... and of cause today one would prefer Vulkan under Linux especially with the open stack (i.e. Linux DRM and Mesa, for Intel and AMD - as unfortunately it is not supported by Nvidia) - but also with Nvidia cards and proprietary driver.
So changing the default OpenGL to Vulkan in start.sh would be a problem for Nvidia users right now ... so currently
editing start.sh can not be avoided for a part of the Linux users.

The graphic problems can be seen as there are 3 pictures attached showing the FF start screen
(all three downscaled for size limits of this forum):
01_*.jpg - Nvidia problem when using Vulkan
02_*.jpg - AMD (and similarly Intel) problem when using OpenGL
03_*.jpg - correct start screen, here using AMD with Vulkan

I had hoped that one could switch the default from OpenGL to Vulkan by default - using starting line in start.sh:
      ./"Forgotten Fields Linux Demo.x86_64" -force-vulkan
But as Nvidia has problems using Vulkan with this game there is currently no way to avoid Linux users to edit
start.sh by hand, unfortunately.

So after creating this post I will inform the developer in the hope that this can be fixed soon.