The game updated and it now shows "testowy build 2" as the version and file names.

The game's devs are based in Poland, as is GOG. "Testowy" in Polish apparently translates to English adjectival "test". So, "test build 2".
Looks like someone at either 11 bit or GOG might've forgotten to give the new build a real name/number before uploading it. (I don't know if it's called the same thing on Steam. It's late, and don't feel like looking it up right now.)

I have it on Steam too from a charity bundle and it received an update a few months ago. There were no patch notes, and a bit of a screw up in that it deleted the game completely. They fixed it, but any question about what the update was for went unanswered.

GOGDB shows the version number as 6.0.8. I can't find a version number for the Steam build.