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.
Let's Play, alexsandrovascocellosdasilva, www.bayatmatin, Tytamice, ChaosSNIPZ, worldoftanksss, seigetsu58, [Hell] Reaper, applepielord333, pokernightpros, Frobro 3.0!, Spawn, [red]폴[blue]츠, KeSSoN, Doctor W.D Gaster, Doctor W.D Gaster, $EXY, Hi, InFinite, Digei123, Froggers, dylanthorington17, WetFarts, Hirasmous, ETA-22, FaZe Snoppbarn, KingShadow, アトミック侍 (GunnerCat), 감귤 작작먹어