Poulpi: For me, deleting the steam plugin folder in C:\Users\<Username>\AppData\Local\GOG.com\Galaxy\plugins\installed\ was not enough.
However, after removing ALL files named plugin-steam-*** from C:\ProgramData\GOG.com\Galaxy\logs, the plugin stopped crashing.
What I did in simple steps:
1. Quit GOG Galaxy (not reduced, completely quit)
2. Remove steam plugin folder in C:\Users\<Username>\AppData\Local\GOG.com\Galaxy\plugins\installed\
3. Remove all files named plugin-steam-*** from C:\ProgramData\GOG.com\Galaxy\logs
4 Restart GOG Galaxy > Parameters > Disconnect Steam integration > Connect Steam
Note : My Steam client was not running at that time (don't know if it helps or not).
Unfortunately, even after authenticating, the client says that I am not connected to Steam.
It doesn't work to me. Steam plugin still crashing to me.