This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
->[MxD]<-, Your_Lovell, Somebody123cool, sidr_12, drumjedi76, Alien, Peluche-Nerv, Mr.Jack, Ryan Soto, Prohorov.daniil2004doge, Alhevi #triggered, re15revisited, re15revisited, Shadow l KryptonNetworks, Shadow l KryptonNetworks, Kayxy, Ack Ack, MuhammadIzzan_fr_, alt1701a, TheGamersDrago, rubimartinbecerracanedo, Ryu