There are 9.402 mods on Steam but only three mods on nexus for GOG. Where can I found more mods for my GOG Version?

You might try downloading the Steam mods to your computer and putting them in the correct place. Would take a little research, but it has worked for other mods. It's up to the people who make the mods to make them available.

Unfortunately, the installation of W&R does not create a mod directory anywhere. Does anyone know where to create the mod folder?

Mods are supposed to be installed in the following directory
C:\GOG Games\SovietRepublic\media_soviet\workshop_wip. The folder names of the mods must have the ID numbers from Steam. So do not rename them.

https://steamcommunity.com/app/784150/discussions/0/4415298372650730377/

Steam mods can be installed with the help of
https://steamworkshopdownloader.io/
and the portable program steamcmd, which does not need to be installed.
https://developer.valvesoftware.com/wiki/SteamCMD#Downloading_SteamCMD

Does this still work? I'm pretty sure the default for uploading workshop stuff now is that it's locked behind a paywall in that you can't download anonymously any more and need to have the game in your steam library.

I'm actually wanting to purchase this game on GOG but the lack of access to workshop mods is making me consider if I should just buy it on Steam (or just not buy it at all).

EDIT: Just installed SteamCMD and it looks like it downloaded a mod without issues. Thanks for the info