Hello.
Where is old version? On gog page is Only Galaxy 2.
I instal it and want back to old one.

I have same problem, don't know where to fing offline Setup of latest version of GOG Galaxy 1.2.xx ...

Here you go:

https://content-system.gog.com/open_link/download?path=/open/galaxy/client/setup_galaxy_1.2.67.58.exe

How about the old Mac version? The installer will not complete, but says there is an error. When installing through a game installer it says it can't find the GOG certificate.

Is there any way to prevent GoG Galaxy to simply reupdate to GoG Galaxy 2?

FAQ Entry 1.2

Which is to say, "officially no" but users have found solutions.

This is another good link on the topic:
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/how_to_keep_galaxy_v12_client_and_avoid_the_forced_v20_beta_update

this older version gives the same problem. Cannot sign in. I have spent many hours now to find a solution and tried to sign in. nothing seems to work today.

the link downloads the old exe from Gog servers and you install Galaxy 1 from that file
sign in = normal after you install Galaxy so that it knows what account to sync i.e, what games you own on Gog

There are dozens of them