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.
L'empreurDuSale, dragonmetal153, colpok, Crazed Mailman, frost, fℓσ | ∂иαℓσяgαмєѕ, Zik Le Chat, Scy, I'm REALLY Bad, Austin.Balmer, |-GTC-| Rob, ᴘяᴏxιᴍιτʏ, |HG| Hova, Larry the Lobster, kralsadasdasdasd, Vexarik, DiViidE, Mc Reggie, Maxime, triforcecultist, Nikolay scp rp, EnErGy, Novagameur, Wasuma, NERO2k, [53rus]*-*VegaZ