Try this one
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/steam_plugin_crashing_constantly/post88

Ty, it works for me. ♥

Hello friends, I received this error when connecting my Steam account with my Gog account, how can I fix it?

'Plugin Crushed'

try downloading and installing the latest (1.0.8 on the current date) version of the steam plugin from github that matches for OS

This worked for me on December 28, 2023. Thank you!

This solution works for me

So, had to do it also today on Jan 25, 2024. GOG Galaxy was very buggy/unstable these days.

Installed ABaumher's latest Steam Integration version (as of now, 1.2.1Beta), following the instructions on its main GitHub page

That solved it, thankfully.
__________
Just to expand the unstable issue a bit, I had trouble not only with every sync, but even accessing the client/logging in. It kept asking me to log in every time, but seems it couldn't connect (and when it could, it always asked me for a verification code and was still unstable). I don't think it was directly related to the Steam integration issue, since after Galaxy regained stability and syncing with other platforms, it couldn't sync with Steam, so had to do as described above.