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.
spaceguy302, viktor_n_hristov, EgorKa, Seanboss360, James West, Savagely Random, johnsailor48, Mr.Wolf, Sufficany1970, wheatlybix2004, jmonte, Rose, Churcher97, ┌Kurz┤, Nom_carver, Jd, LazerSpy, Syntax