I recently purchased Project Zomboid and would like to install some of the mods that are available. But every source I go to has instructions for installing with the Steam server but nothing for GOG. Would appreciate any help that could be provided.

It's quite easy.

1. Download the mod:
a) from a know source
b) from Steam using https://steamworkshopdownloader.io/ (just place the link, download and extract)

2. Place the mod (it's a folder) inside the project zomboid mod folder: C:/Users/YOURNAME/Zomboid/Mods. The game allows you customize the mod folder.

3. Go to the game, select MODS from main menu, and enable the ones you want (if you dont see them, make sure the default mod folder that PZ shows you is the same folder where you placed the mods).

Cheers Liquid, top link, great stuff

Valve made steamworkshopdownloader.io shut down. :(

Workaround:

You can do it manually using github.com/shadoxxhd/steamworkshopdownloader

But READ THE INSTRUCTIONS (so you don't ask dumb questions about how it works or where the files went)

Project Zomboid's Steam ID number is 108600

You'll have to get the Steam workshop links to items individually using a browser and paste the links into the box in the software.

My 10c. Because the "downloader.exe" does not want to work on my OS.

Google for "steamcmd.exe".
Takes you to Valve Developer Community.
Download the ZIP file.
Create a folder somewhere really convenient, copy ZIP there, and extract it.

Run the .EXE.
It will download a lot more stuff.

Joy of joys, you cannot copy-paste into this program. How wonderful!

Type "login [username]"
Enter your password -- nothing seems to happen while you are typing, but just press enter when done.

Some more stuff happens. So mysterious...

Next type: "workshop_download_item 108600 2276669022"
Where the last number you get from the www workshop page as an ID. In this case, its a fanny-pack mod. So maybe go get something else instead as your test-case.

Wait.
And should now find the mod extracted into: "..steamcmd\steamapps\workshop\content\108600"

HOO.RAY.!.!.!