I downloaded / reinstalled the latest version from GOG and run it using the GOG Galaxy launcher. Just launching the Galaxy program results in the 'GOG Galaxy - Error' popup, before I even try to launch SS2.

Anyone else having this problem? I submitted a support request a few weeks ago, but crickets so far..

So have you tried you know, running it without Galaxy?

HA!

yes, double clicking on it directly has no effect, like it tries to launch but doesn't.

Hmm, alright. How about this: Open Terminal, point to the file, and see what the output is.

Regarding your error I dont think that is specific to SS2. I have had this error twice the last week and it can be temporarily be fixed if you follow the instructions here:
https://www.gog.com/forum/general/cannot_open_gog_client_on_mac_connection_problem

Regarding System Shock 2, I have the same problem where nothing happens if I launch it. With or without Galaxy. Trying to drag System Shock to Terminal and run it gives me "zsh: permission denied: /Applications/System Shock 2.app".

Just noticed that they solved this problem...by simply removing any mac mentions from system requirements.
"It's Windows or nothing from now on, buddy!"

Well, considering that Apple is a closed garden ecology that keeps shrinking, shrinking, and shrinking, can you really blame them?

This has to do with Apple dropping 32 bit support on macOS. See the following tiny note on the system requirements on the game page:

Mac notice: The game is 32-bit only and will not work on macOS 10.15 and up.