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.
Alpha Networks, Chantalia Queen, Stacks, [FranceRp] [Gmod] [LilianYTB], Chrèdeur, chauKlet, wortartol, Yumiro Pastoche, Devilitec, Греча, Lucas Ismal, NUDIXAKO, NUDIXAKO, boyhec, mnafek aymen, Tikomatura, Tim, [Fondateur Jmods]jonathan123jojo, fsociety, Neizoo, Throbbin Hood, [FONDATEUR] Thomas McGregor, "Hawk-Eyes" Mihawk, 3499387031, Jumpy, №︎Ⓢ︎ ℭ, CivilGamers.com - ItsJustDamian, Narnia/Kiwi, Doc Scurlock, DowntroddenTV, [Owner] Jack, Remember Daily, DJ_Tobo, GhostArmyZ, BranX, || HarzeL ||, Name, L1L1pyt, L1L1pyt, Trippyy, GoDz_GodHasFaith™, Papa, DrMeepso, Lucky, Younes Bnbm [Fonda-RockfordRp], SamuTheFrog, HeraLD, hokage, ass csesport.com, Λ | Andreas34 | +1564 others