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.
BYVALYI_FRONTOVIK, rudionMT, Juba, OSITO (TED), sumo25471234, acsrefe, Waifu_Whaler X, dima, Popeyedslugs, Terminator, louni.younes, Sigalaeff0607, SounDemoN, Czer, Czer, ilee57145007, xxevolutionxx27, urialia, Kōgō Tepes, seriou*****, Shadow ☕, filipka.ivanov.2016, ranpar1972, adenildacsme, riybokon, sam1995agent, Genus, bael, ⒽΞ⋀DR00Ⓜ | CSGetto.com, Crenegation121, MannequinneD, 蛋疼的一天, falseprophet141, Olddd, lionheart617, Meet the Joves, Dav3, Dav3, chantry.conkle, novaprospektt, tindail, albertwesker9999, Beep Beep Evolution Party, thingstewart, Master140, 구구구, Meatstick, Gabe, Moostic, Gorgeous +92 others