What is a User Created Map?Edit
Sources of User Created MapsEdit
There are two main sources for user created maps and content. One of them is the Steam Workshop, this service (provided by steam free of charge) allows you to 'subscribe' to content. Once the author of the item releases an update to their content, it will automatically be downloaded to your Garry's Mod.
Another source, is Garrysmod.org. This is a community site that hosts a large number of user created content. You will need to sign into the site using your SteamID to be able to download the content.
Files downloaded from Garrysmod.orgare downloaded in a (compressed) .zip file. These will have to be manually installed into your Garry's Mod directory (default C:\program files (x86)\steam\steamapps\common\garrysmod\garrysmod - If you're installing a map, you'll need to go to garrysmod\garrysmod\maps). You will need to restart Garry's Mod (if it was already running) after you've installed mods.
This is the list of user created maps. Updated 1/23/2014.