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.
szeppatrik14, Camiloxio, teucro75, eugenio.kyle1219, Pixans [TF2BC] #savetf2, phongsaphathpo, agrenademon, NORDUK, ThermalVision, experiment 626, erickhdez035, A barely living kickball, NearWanna, Megagecko, LeBronJahames, LeBronJahames, Haniball, oussama85b, oussama85b, cipher192, NlxRoberT, Cyrax, nikita.ryabcev29, Nix, XxBLIZORKxX, squierfield, Daddyhoward, elrafis2345, ROCKA, deme.angelo, FatherKilla, johndoesthrowaway1964, jms001227, Blackstar, Alpha 1701, Fr0z1q, cesar858, xVexezle, x_broken_trustt_x, Angat, srmolotov0, majorkombatfnaf99, NySky, Ramirez, CTHULHUGAMING9000213, mmochi1008, 〘ꕤ〙 Moneko ! 数, ShutBullShit, indusara59, Remartant +1637 others