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.
범땡이, Ichigo Kurosaki, MaxI, Super Courier, Super Courier, atomicfox15, [LFG] FallenDj122, LucapioneTV, Ben Dover, ☢ [SpooderKing] GaMeRoId88 ☢, ⓜⓐⓚⓢⓘⓜⓔⓝ, Kingram, sagar, cheschess, FroSt Panda, HorseFighter, [AG] ItsSintex, Cien_, ElmoIsNotFunny, ☭✞TK MEM3 ☬ | Proxy net., Some, RGamers, Theitalianmango, SangTheBoy, Starfy, [Mythical Gaming] TheDemonicPie, ߷ItsYaBoiZero | Trade.tf߷, Nekros, Skit, UndeadHero, KRYTOX, ΩFreakΩ, Bakari black, ★RedPanda★ kickback.com, Mippy, cool cunt, •GrandBoss•, Ace of Kings, Akari_, [RFAI] Strayed, Alukard, DeadlyFlareon, Doc Brown, Goof., [RFAI] CptDashie ✅, TVL Venomous, Liquid v2, Awen, DasBoSchitt, Vault Sniper +110 others