Instructions
Double-click the ZIP file that contains the GMod files that you want to install to unzip its contents. Click on the add-on folder in the new window that opens to highlight it and click the "Extract To" button at the top of the window.
Click the "+" icon to the left of the "Local Disk (C:)" option, followed by the "+" icon to the left of the "Program Files" option.
Click the "+" icon to the left of the "Steam" option, followed by the "+" icon to the left of the "steamapps" option.
Click the "+" icon to the left of your username, followed by the "+" icon to the left of the "garrysmod" option.
Click the "+" icon to the left of the "garrysmod" option within the "garrysmod" folder and click the "addons" option to highlight it.
Click the "OK" button to extract the files to this location. The GMod can now be used in your Garry's Mod game.