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.
Ivan_kos, iliashishkin.pro, Lv 60 Healer, [ORP] AurGaming | Edward Ikaris, Shawdy, Warcraft, BaraSHek, SNG | Panda™, wailik2003, Galagan, Col.Sam, BlueGhostHD, EgN| Danker Pepers, Cipher, FranDM, zfadarkness, joawnai, Corvezeo✅, ❤ C R E O N I X ❤, [MG] Jackson169, Johnny, ╰☆╮ sarah ╰☆╮, LittleBigBug, Captain Phillips, Mike, Huskybusky, |W-L|Sloupock#, Stuuuuuypid, Hayes Miller, TG4L|Aaron|Croncos.com