"Mini Metro" got a Linux update on GOG on 29.10.2020.
Unfortunately, the "start.sh" is completely messed up.
It does not change to the subdirectory "game", the current binary is
no longer "Mini\ Metro.x86" (32 bit) or "Mini\ Metro.x86_64" (64 bit) [both about 26 MB],
but only "Mini\ Metro" (a 64 bit only binary) [of tiny 6 kB] using a big shared object "UnityPlayer.so" [of 30 MB size].

One can start the game from installation directory on CLI via the line:
   cd game; ./Mini\ Metro; cd -
or add the following lines to "start.sh" just before "# Initialization":
   cd game
   ./Mini\ Metro
   cd -
   exit
Thus the rest of the script is not used.
One can invoke it via
   ./start.sh
in the GOG directory or invoke it with full path using the preferred DE.

I will just create a ticket at GOG support and ask them to trigger the product team to change their start script.

The call for the GOG support got forwarded to their Linux Tech Team on 31. Oct.2020.
This seems to result in the new version 202010161452_release_43_g_42274 I spotted
today (03. Nov. 2020), as the start script now works without any changes.
Thanks to the people involved so all gamers can enjoy the new version under GNU/Linux.