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.
Bunneh, Archmage Frostivious, Flixy, Jonas Drain, ✿ Princess Peasants ✿, Xeno, Baron Samedi, T!nx, InvalidFish, Taqza, tirosgamingyt, Töasty, SmallMischief, brancomeister, snell :^), Sabun, Ronsan (I am the law...ng sword), [KOR]혼살, dizla, Grinny | AreteGaming, damon5, Kat, Natolikin, =RGS= Mr.ROFLMAN™, Yashirmare, ℭ℘τ. Roag