Please, keep old GOG safe, DRM-free and for offline play, remove the galaxy client from the installer. Download it should be a voluntary choice.

If you're downloading offline installers, in theory you have the "choice" if there is a bundled version with galaxy : to see what GOG call the "classic version" (= without Galaxy bundled in the installer) and being able to download it, you have to click on "more", and here you should see "Classic installers". See the image I put to show you the exemple with Stardew Valley (I don't own Stars in Shadow :P).
Hope this helps !
(I discovered that thanks to the forum and Stardew Valley that I own)

Well, yes, I didn't know this. Thanks, man, and vive la France!

Well, there wasn't a big communication from GOG about this (mainly this post so... thanks to the forum users :P)
Ahah, you're welcome ;) Viva España ! :D
Oh, by the way, another thing about which there was a "lack of communication" : check your chat settings ;)
And if you already don't know this script, take a look at it, it changes life : Adalia Fundamentals.

Didn't know about the chat settings, thank you for that!

You're welcome ;)