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.
Max159, ErenSkn, XF, justinee.serano, (C3)matthew47m, Yneomé, Анансовый ананасекс, #VACation#HARVEST, ///Nyma///, CatNike, legamercoc322, [AJ]Art8mka, Brother, Sound_Extreme, DaddySinR, [GOG]Ukitake, [SMOX-RP] Stim85, Gulfik, Horo, Rossi - Gone, Твоя мамка, GK, ExOn | Douille's, ｅｌｉｔｅ, MaMka SkaZaLA ya HARVEST, docdo1, Michael Deets