Tynan did enable anonymous download of RimWorld-mod's from steams workshop.
https://ludeon.com/forums/index.php?topic=50850.0

In other words. it is now possible to anonymously download any RimWorld-mod using the steam command line tool provided by Valve. That it SteamCMD.

A brief description of how to download a mod with this tool follows:

1) download SteamCMD from Valve's official site.
2) find the AppID for the program. Look at the link while browsing the app on steam RimWorld has 294100
3) Find the ModID in the link while browsing the Mod. In this Example The ModID is 735106432
4) run SteamCMD and wait for it to download the command line version of steam. then use the commands:
login anonymous
workshop_download_item <AppID > <ModID>
Using the example above is wold look like:
steam>login anonymous
steam>workshop_download_item 294100 735106432
5) You can now find the mod in:
...\steamCMD\steamapps\workshop\content\<AppID>\<ModID>
or for the above example it wold be:
...\steamCMD\steamapps\workshop\content\294100\735106432

I have tested this with the above example and it worked.
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A somehow longer description is here. (I learned it from this)
And finally a YouTube guide showing download of a mod using this method.
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One might perhaps use https://steamworkshopdownloader.io. Because I guess that this specific site happen to use the above method. But i will not promise anything. Just remember that most sites claiming to download from steams workshop do not work reliably. The manual method shown above do.

Agrilla, thanks for taking the time to write this! If there's any GOG employees reading this, please sticky this thread.

Recently I have integrated mod download funcrion with SteamCMD to RimPy Mod manager. It may autosetup it and download/update mods with a GUI. If you are interested, check it out. It is free.

I was just looking at this as i bought rimworld directly from sendowl's main website. I'll see if it works the same way. If so sounds like a wonderful way to get around the mod gui issue that is mainly primed for working with steam (but can work with the gog and standalone build from their site) that loads mods when put in the appropriate mod folder.

For anyone interested here is a new open-source workshop downloader github.com/VovoloGames/WorkshopDL which includes a ton of features like collection support and multiple download providers and more!

Now that Steamworkshopdownloader.io is gone here is a GUI interface for SteamCMD

https://github.com/shadoxxhd/steamworkshopdownloader/