Things You'll Need
Instructions
Navigate to garrysmod.org (see Resources), the repository for custom Garry's Mod content. This is different from garrysmod.com, which is the homepage of the game.
Browse and download whatever seems interesting. Remember, having a theme will help your server be popular and stand apart from others, so download only those things you think will fit.
Upload your content to the server. For rented servers, this will be through FTP or a Web-based control panel. For a server you physically own, simply copy and paste. Nearly everything you download will go under "garrysmod/garrysmod/addons/," though maps will go under "garrysmod/garrysmod/maps."
Restart the server so it recognizes the changes. When new players connect, they will download all of the content that you just uploaded, allowing everyone to take advantage of the extra content you chose.