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.
Chori, enzoerine1984, IceHunter333, ILoveCookies101, ZAWMBEEZ, dulindawid2 KeyDrop.com, zmbymstr11, Gregoose, vermaabhinshek1009, cristpohermedina21, Sех, ♥ yeks89 ♥, twin_freerunners, c0ff1nqu33n, 우지해, LEGEND, SárKány, SwampF0x45, nopayday25, Mind_Trip, aujuget, Goldguy0710, edgarsgrivins, edgarsgrivins, issiah06, Mikevioleteyes, Papa Gregory, copemichael58, Linkinbangz, oliveralay8, lane10b_e297w, 後 B Witch, Vertex3x, Pecan24, matiasgabriel98, reinaldomoralesmasyrubi, kREGORY, barrios99093775, MellowgolfTheZH, MellowgolfTheZH, BluePower657, otagariesodk, merchantmikemc1, rdude6, horrid2thecore, wils.art.hall, joosephmanso, joosephmanso, alanskywar, mati.beltramelli +1284 others