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.
postal.dude.is.gay, Danipp, 87392, [nBn] Beebs, t_rexfire, t_rexfire, Souta642, Neatu, romerofco75, sanya18881, wefkwkfweewk, Smokey, carguakenneth926, danielflupaixao, titanhoss, LunGOD, lienhoankich, Joker, hoangtuanphong208, Stalker171, Green, _maoe, idontknow, licedor817, carloischayrez17, freakyusx, Cereal Killer, Megadöll, Megadöll, Megadöll, kingcold22lol, Ricktorical, 山上的千里眼 もみじ, frksurf, Musicman626rh, FromMyFinger, Suikodan, chujio, Jafari "Bongo" Ongoolo, cirvis4999, nightpeep, mardih2019, mardih2019, Bennett Source, TurnBased, ruben.leal0826, alemandom04, ทรงพระเจริญ, tuancuaong1981, doguinho caramelo +1248 others