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

Following up on this a bit, what changed is that you cannot use 3rd party Web API to download mods, but SteamCMD is provided by valve and works fine as you found out. You can make it more user friendly by using WorkshopDL:
https://github.com/imwaitingnow/WorkshopDL

2nd, I got bored of trying to figure out correct way to set up the workshop_wip, so I just use JSGME and let it overwrite files in the media_soviet folder. This might be because I want to use the music mods and those need to be manually installed anyways! In any case, JSGME takes backups of any files it overwrites so uninstalling the mod is a snap. JGSME goes into main game folder, mods go into MODS folder under it, and anything below mod folder gets copied into the game folder as-is. Image maybe makes it clear?

As a bonus you can give the mod folders a descriptive name so no need to keep in mind "3247412300" stands for Kola's socialist music pack..