Garry's Mod uses Steam Workshop addons, which include maps, gamemodes, weapons and tools, loaded together through a Workshop collection. Addons can also be placed by hand in the addons folder.
Back up first. See How to back up and restore your Garry's Mod world.
Add a mod
- Create a Steam Workshop collection and add every addon you want to it. Set it to public or unlisted, because the server cannot read a private collection.
- Copy the number at the end of the collection page address.
- Paste it into the Workshop Collection setting under Settings, then Game Settings, and restart the server. A single addon id loads nothing, it has to be a collection.
- The server downloads and loads the whole collection at every start, and players get the content when they join.
- To add a custom map by hand instead, stop the server, open Files, go to garrysmod/maps and upload the map's .bsp file. A map with the same file name replaces the existing one. Then set Map under Settings, then Game Settings, to the file name without .bsp, and restart. Players who do not have a custom map must download it when they join, which can be slow or refused, so putting the map in your Workshop collection is the better route.
Where files go
Open Files and use these places:
- garrysmod/addons (inside Files)
Check it worked
Once the server has restarted, read the Console. A mod that loaded shows up during startup, and problems show under Errors and Warnings. See How to use the console and run commands and How to add mods, plugins and modpacks.
If the server stops after you add a mod, remove the newest mod or restore your backup. See How to fix a server that will not start.
