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.
◘[Tes]◘, daza_gharaibeh63, bloodbathbowl, daedae1496, koossa12344, Marumo, The Programic Knight, Cocoa Bean, Cocoa Bean, Cocoa Bean, ECHO-6251, Flamer, Goat Of Time And Space, Super Sand, Lincoln Balloon, Lincoln Balloon, Time Turner, SenseiStereo, Beeblebuice, Kiln, ashtheslimedragon, KABE_13, odama10, michael63963, Tony Tagliano, MrMcFeely☣ᶠᶸᶜᵏᵧₒ, Chibi xXxScrublordsxXx, jordanscurlock, kyekyekye, Dr. Cooldude, President Erkoc, [Zeus], Hambo, WPN, WPN, WPN, WPN, MagicalFruitBasket, MagicalFruitBasket, Agathon, Thexant, Buzzofwar