If you don't know how to add the map, then read this.

Step 1. Download the link
Step 2. Open files.
Step 3. Go find program files (x86), -this could be on your (C:) (D:) or whatever drive you've installed Steam on- > Steam > SteamApps > Common > Garry's Mod > Garry's Mod (yes twice) > maps
Step 4. Copy the map you downloaded to the maps folder and you will be able to play the map
Step 5. Have fun!

If you don't know how to add the map, then read this. Step 1. Download the link Step 2. Open files. Step 3. Go find program files (x86), -this could be on your (C:) (D:) or whatever drive you've installed Steam on- > Steam > SteamApps > Common > Garry's Mod > Garry's Mod (yes twice) > maps Step 4. Copy the map you downloaded to the maps folder and you will be able to play the map Step 5. Have fun!