Is it somehow possible, to get older versions back, maybe via this Galaxy thing? I have the need to go back to 2.8.

In gog galaxy it should be possible to go at least some versions back (but not all the way).

Try from the game page in galaxy-> More -> Settings -> Override features

Thank you for quick response. There is no option to get older version in Galaxy (btw. what a piece of bloatware.... omg). I just should have been smarter and archived the old versions. Found also this information:
https://www.gog.com/forum/stellaris/is_191_version_available_to_download

Galaxy only holds the last few versions and Paradox brings out a new random patch every few weeks
at the time of this post the current build 3.0.3

however you can use the Paradox key to unlock their build i.e, the one Paradox store sells dirrect and downloading that allows you to use any build of Stellaris... they keep older versions for download

notes;
Paradox, Gog and Steam builds don't play nice together and having one installed on your system means the others will not work i.e, you have to actually uninstall them from Windows before installing the other or the files will be buggered up

using your Paradox store key that Gog gives you means you must have a Paradox account and they lock the key to whatever email address you give them when you build this account i.e, Paradox is NOT drm free

its worth pointing out that installing 2.8 means you will be unable to play multiplayer against anyone with other newer builds of Stellaris

Thank you, ussnorway for outlining all the options at hand. As said, sadly I was not smart enough to archive the old standalone GOG installer. Its all gone, like tear in the rain. Time to... adapt. Sooner or later, I will enjoy playing the Dick.
And no, I will not fall back to some DRM infested Paradox account. I am happy that I can get my drugs here at GOG, even if they dont provide the best stellar service that I can imagine.
Alright, back to anomalies and archaeological sites... and those nasty fanatic purifiers in my backyard.