Many Steam games support the steam workshop. It is an easy way to share community maps and addons and is very useful for custom servers, without needing to worry about setting up FastDL. LinuxGSM adds workshop pre-configuration to game server scripts as when available.
You will need a Steam API key, a collection to subscribe to (you can create one), and some config parameters in your LinuxGSM config.
Simply go to https://steamcommunity.com/dev/apikey and follow instructions.
Go browse collections for your desired game, and click "Create a collection".
Add some addons to the collection, then publish the completed collection. Then get the collection page id which can be found on the page URL. For example:
The collection id would be
Edit the LinuxGSM config
For Garry's Mod, edit those 2 lines accordingly in your LinuxGSM config
For CSGO, have a look at these lines in your in your LinuxGSM config