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.
penandspeakers, HappyClown™, Locuno02, Lieberman, Toocanzs, OneOfHenry'sMen, Arturhk, antoniusevans93, Schne1der, ♠ Ĭ₣℟Ì₮ ♠ [7E], ♠ Ĭ₣℟Ì₮ ♠ [7E], rocketstrong, rocketstrong, Ronnie Paints, Legion Riven | RIPPER, °•.W.D. Gaster ☝✌, James, [Locust] Grub Smith, japanrailway90, sagar, gabrielucho1, Franklen12, renan3989, NYPD, KARAKANANON XD, muhammedalgul_23, Johnny, dsyfguf2423, #R.O.G Potter <|DXRacer|>, ᵀᴴᴱ Z | ε L α S CSGO-SKI, (피해자)mr.comd, Ⅸξ.UMR ˙, ּ Luke, 한국여우, 더기shin.DUGI, DARKNINJA349, VERTual, minired, YongLi, john from the government, AlphaOmega, AlphaOmega, Lord Empty Bucket, Eden, [BA] SammyT291 [FT], Sh e Sh, *_ZeHiR_*, aleksansander, babaarda2005, Opo25 +126 others