workshop + fastdl

There's another way to go about this. This method forces all clients to download selected addons onto their computer, so they won't see errors anymore.

https://wiki.garrysmod.com/page/resource/AddWorkshop

Make a file called workshop.lua in your server'sgarrysmod\lua\autorun\server. I recommend using Notepad++ for this.

Then make a line with the code
The workshopidhere refers to the workshop addon ID.

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After that is done and you want to add more, keep adding new lines according to the workshop ID of the addon.
Hope this helped!