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.
JessieGaming, Reemthepro hellcase.com, Al Shebab, Gangsta Nation, xProGamer45, AwayJokeR, SinsPatron, Marcel Zukini, [qRP] ethio, Ronnie Paints, Sinon, =RED=, Alice Henderson, Caver, NoName, BrOkEn_SaNiTy, НАГИБАТРРР228, ⠀ᶠᶸᶜᵏᵧₒᵤ⠀, CheeseZ, Grubasek!, Akio1337, Wcar, THEGAMER123FTW, naucy [FR], Prime575, pony, LaImE, red the punnyone, ⓜⓐⓚⓢⓘⓜⓔⓝ, b.talkingparrot, FireXoNoX, eliseev-valyusha, Blobfish, Gamer2001, Ash Don, C2S01, (chef helpeur Jean Trude Off.), Yolapatatoes, sagar, Cj(Communist Leader)-gts, Loganus the great, ansueh4, FB||Dry_Bones, FroSt Panda, Meowy, mr.timbs, AncRad, , [LSTA]blc1234, Lordwinkles +1089 others