Instructions
Download a mod for "GMod (see Resources). Find the download location of the mod. You can find the downloaded files either by opening your Internet browser's "Downloads" window, or by searching for the file name using the Windows search function. Mozilla Firefox downloads can be found by clicking "Tools," then "Downloads." For Internet Explorer, check your "Downloads" folder.
Extract the mod files to a folder of your choosing if they are compressed.
Open a Windows Explorer window. Navigate to your "GMod" installation folder. If you can't find the "GMod" installation folder, right-click the shortcut on your desktop and click "Properties." It gives you the file path information there. Use that file path information to navigate to the folder. The "Garry's Mod" folder is normally located in the "steamapps" folder.
Follow the instruction file for the mod. You may have to copy and paste some folders into your "Garry's Mod" folder. You may have to paste the files into the "addons" folder. If you are pasting folders into the main folder and are prompted with an overwrite request, allow the overwrite. Existing files in the folders will not be changed.
Run "Gmod." The content from the mods should now be available for use. If it isn't, double-check the installation instructions as well as your extraction location to make certain you didn't miss any files.