Luckily I tried the 2.0 beta before and was aware of all the issues, so I do not want to update to 2.0 but I get that popup every couple of hours, wondering if there is any way to disable it?

My bad, answer in FAQ, rename C:\ProgramData\GOG.com\Galaxy\redists\GalaxyUpdater.exe. (don't see the option to delete this thread but I miss so many things already so who knows!)

Thank you ! I don't want to upgrade to 2.0 because I can see that it is a mess. I just need to Rename it? Can I just move the .exe or delete it ?

It's possible to delete the .exe file or just change .exe to .old or something. For future-proofing reasons, it's generally recommended to keep the file with an edited name so you can just change the name back if and when you decide to update in future.