Instructions
Open the folder containing your downloaded map. This was specified when downloading the file.
Right-click the file and select "Extract All" in the right-click menu. Wait for the file to extract and open a new folder.
Open your "Garry's Mod 10" installation folder. This is in the "steamapps/USERNAME/garrysmod" folder within your Steam installation folder, which was specified when installing Steam.
Double-click "garrysmod," followed by "maps."
Drag the extracted map folder into the "maps" folder to complete installation.