So, the Mac version isn't listed because it was broken by an OS upgrade a few years ago. However, you can still download it if you bought it, and it can be made to work flawlessly on new versions of the OS (I'm on 10.13.6 at the time of writing). This requires some moderate technical knowledge, but is pretty straightforward. It could no doubt be automated by someone sufficiently motivated, and GOG could easily update the app and directly sell the Mac version again if they wanted to. (But they won't.)

Start by downloading Spacechem. Use the "download offline backup game installers" option; I don't know what happens with Galaxy. This way you get a nice, functional .dmg (none of the .pkg nonsense GOG does now). There's also a file you'll need on Github, called bonsai.tar.bz2, available here: https://github.com/leafi/fix-spacechem-os-x/blob/master/bonsai.tar.bz2 Download that too, put it on your desktop or something, and unzip it. This makes a folder called "bonsai". It's a fix for Spacechem, but the fix ironically needs to be fixed, plus it was made for the Steam version, so we'll have to do stuff manually.

Open the Spacechem .dmg and drag the Spacechem app somewhere temporary like your desktop, then eject the .dmg. Right-click on the Spacechem app and do "Show package contents". Navigate to Contents/Resources (easiest in list view). There's another Spacechem app there...yes, an app in an app. Because of reasons. Right-click on this app, do "Show package contents" again, and open the Contents folder.

Now open the bonsai folder you unzipped previously. You see Frameworks, MacOS, and Resources folders. Drag the Frameworks and MacOS folders into the Contents folder from your Spacechem app-in-an-app, replacing those folders. Do NOT do this with the Resources folder. Because doing that will delete a bunch of stuff, and you'll have to start over. Instead, open the bonsai Resources folder, select all the files, and drag them into the Spacechem app Contents/Resources folder, replacing those files (do "Apply to all" when asked).

Now for the part where you need to use Terminal: open a Terminal window, type "cd " (without the quotes, note the space), then drag Contents/Frameworks/SDL_mixer.framework from your Spacechem app-in-an-app into the Terminal window (which makes the rather lengthy path for you), hit return. Type "ln -s Versions/A/SDL_mixer SDL_mixer" (without the quotes) and hit return. This fixes the borked symlink. Also that's a lowercase L, not a capital I. OK, so probably just copy and paste the stuff inside the quotes instead of typing it.

You can now drag the Spacechem app-in-an-app somewhere permanent like Applications or wherever. You don't need the "outside" app, just the "inside" one. Get rid of the shell/"outside" app, since it's just junk, and the bonsai folder. The fixed Spacechem app isn't signed, so to run it, you'll have to right-click and select "Open", then click "Open" when it complains about unidentified developers. You only have to do that the first time; after that just run it normally.

There, working Mac Spacechem! At least it works here. It's possible you might have to do something else...when I was investigating this, the relevant forum topic on Github mentioned some changes in the Tao.Sdl.dll.config file in Contents/Resources in the fixed Spacechem app. I didn't have to do it, but if Spacechem still fails for you, give this a try: Open that config file in a text editor, delete the contents, and replace it with this (copy and paste):

<configuration>
<dllmap dll="SDL.dll">
<dllentry os="linux" dll="libSDL-1.2.so.0"/>
<dllentry os="windows" dll="SDL.dll"/>
<dllentry os="osx" dll="@executable_path/../Frameworks/SDL.framework/SDL" />
</dllmap>
<dllmap dll="SDL_image.dll">
<dllentry os="linux" dll="libSDL_image-1.2.so.0" />
<dllentry os="windows" dll="SDL_image.dll"/>
<dllentry os="osx" dll="@executable_path/../Frameworks/SDL_image.framework/SDL_image" />
</dllmap>
<dllmap dll="SDL_mixer.dll">
<dllentry os="linux" dll="libSDL_mixer-1.2.so.0" />
<dllentry os="windows" dll="SDL_mixer.dll"/>
<dllentry os="osx" dll="@executable_path/../Frameworks/SDL_mixer.framework/SDL_mixer" />
</dllmap>
<dllmap dll="SDL_ttf.dll">
<dllentry os="linux" dll="libSDL_ttf-2.0.so.0" />
<dllentry os="windows" dll="SDL_ttf.dll"/>
<dllentry os="osx" dll="../Frameworks/SDL_ttf.framework/SDL_ttf" />
</dllmap>
<dllmap dll="SDL_net.dll">
<dllentry os="linux" dll="libSDL_net-1.2.so.0" />
<dllentry os="windows" dll="SDL_net.dll"/>
<dllentry os="osx" dll="../Frameworks/SDL_net.framework/SDL_net" />
</dllmap>
<dllmap dll="smpeg.dll">
<dllentry os="linux" dll="libsmpeg-0.4.so.0" />
<dllentry os="windows" dll="smpeg.dll"/>
<dllentry os="osx" dll="../Frameworks/smpeg.framework/smpeg" />
</dllmap>
<dllmap dll="SDL_gfx.dll">
<dllentry os="linux" dll="libSDL_gfx.so.4" />
<dllentry os="windows" dll="SDL_gfx.dll"/>
<dllentry os="osx" dll="../Frameworks/SDL_gfx.framework/SDL_gfx" />
</dllmap>
</configuration>

Apparently that forces Mono to use the included SDL libraries instead of the system libraries, but like I said, it wasn't necessary for me. But it doesn't hurt if you do it anyway.

There's a working 64-bit version now, so this topic is obsolete.

Great news )