This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
snyxnon, Helladella, aaaaiio2312, jay1_z, BERSERK, jaymalcolm25, _-ReApEr-_, bibrkacity, Rouge.Exe666, kristaps6484, CatDraculaX05(Good Vibed), itream30, The Empress, risoskol, origio, Krobus, Krobus, XDkarolis, DraculaCV, galusins56, Legal_name, elfrikidecaballito, Zachariah, Zachariah, Loiro, Rion, Gülümse Sıkıyorum, Crunchidile™, Aeon, Mister Fael, Famineskull98, Rein, Bombstrap, LuisMexElite