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.
Hive365 | Demo -™, bashir, grimravus, Littlefoot, ahmedalfalahy2001, Englandball, Windows, pedroelias0712, jackbiles, FIRE GOD, Razzer, Cute_Bonnie1987, Vovanbl4, FuriousRedstoner, bertisis, civil rights leader flavor flav, Vandarxx, [FL:RP] Darkside, olivierchausse, olivierchausse, Marisa Kirisame (Cc), Dune Jumper