Before we begin, you will need to acquire a Valve tool called SteamCMD. You can download that here.

Once you have downloaded and extracted the zip file, you will be presented with the file SteamCMD.exe. This is the program we’ll be using; make sure it's in a folder of its own and run it, and you’ll be presented with a command prompt window. If you've never used SteamCMD it will run a series of updates first.

The first thing you’ll need to do is log in. If you have a Steam account then great, you can use that, but this tutorial will guide you through logging in anonymously. Type the following, then hit return:
The program should respond with the following:

If you have an account you can use the following:
SteamCMD is supposed to allow you to enter your password separately but that currently seems to cause it to freeze, and for obvious reasons entering your password plaintext in what could be a room full of people is less than ideal. Logging in anonymously is quicker, easier and more secure so I'd recommend that.

You now need two numbers; the AppID for the game and the Workshop ID of the mod you want. The game's AppID is 624970, and you can find the Workshop ID from the URL of the mod's workshop page like so:
The ten-digit number where it says "?id=" is the number you want. So, for this mod, the AppID is 624970 and the Workshop ID is 1133953644.

Now you can return to SteamCMD and tell it which workshop item to download using the following command:
Which of course in our case is...

You should see the following confirmation message:

The attached screenshot shows my copy of SteamCMD just after completing of the above download.

The exact directory will change based on where you put SteamCMD and the filesize will of course vary based on which item you downloaded. Either way, you will now need to navigate to the folder you've just created - if you put SteamCMD in a folder of its own, you should easily be able to locate the /steamapps/ folder that is now alongside it.

Each mod you download like this will create their own folder within the /624970/ directory. They'll each take the mod's Workshop ID as their name, but be warned that renaming them will break multiplayer compatibility. To move them into the GOG version, simply copy-and-paste the mod's folder and all files within it to the gogWorkshop directory in your My Documents folder, which you can find here: