Call it beta or test doesn't matter it should be either public or non-public (few selected testers).
The "btw. here is the beta passwort for everyone that has bought this on Steam" was, in my opinion, not only half baked "public" but literal trolling everyone that not bought on Steam.

But (hopefully) *was*.

the 8.004 version has been on gog for months now. the newest 0.8.017 version streamers are posting about isn't updating in the Galaxy Client. I did see GOG added a sale price to ML and added for play on LUNA. So the Devs of the game might only be dropping future updates to the game on DRM clients like steam/luna. Which is sad. ymmv.

Current version at GoG 0.8.024
Current version at Steam 0.8.024

This update has been released on both platforms at the same time.

I've also checked the history: There has only been one Beta Update on Steam (0.8.017) since 0.8.004. So, GOG users didn't miss anything out.