Put the .bsp (map file) into your maps folder, you can reach this by following the path:
steam/steamapps/[your name]/garrysmod/garrysmod/maps
Enjoy, have a look around and see if you can find the secrets placed in the map, one is through a 
teleporter and one is through noclip (you will need tf2 for the noclip secret).
Have fun! :)