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.
Ronnie Paints, Family_wms, davidwerner007, 화이리 (ファイリ), babaarda2005, Tradedoodle, adityasurya1434, Deaf Kikbow, Scourge, Exonrades, Exonrades, radin-f, balkaptan, SKY277, sandiegoklassik, HailBolbi, Tricky, pitzo, Mr.lemon☃❄☃, ggggg, ✯ Shark, Apex Predator, illuminati, M_Farhan_A, Orcssasin, gs_umutcan2011, wildchee, kyle.stokes321, Uros Uki, 감귤 작작먹어, sumeetgupta501