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.
Guillerayo.gs, GustavinhoCraft, kbubuka, ivaha07, tai20041, [FTL].BiFiKs, busermanov5, Andersso, areeshim, Wukimos, robbiesansores, Arthur, Sergiokun4, TEHDIAMONTRULL, arken, ELWC» DoneurCreations #Horizon, rguezrguezfran1, Nikaidou // 부대, Lucasgold, โอเลี้ยง TheVoicekids 4, Mertopix