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.
PALU, Abola = Cancer, petersr12345, pokemonbeginner6, TomaToe, Amigos11, COAL, locatelliandrea, renoxuk, Eduardoevangelistagames, BlonkoGoesBOOM, zanibalmarifawi, goood, ayla.gunes.akdag, salo_S94, kib04285, Yuri, Commander Shepard