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.
Benker, HORNY GAMER, アルケー, ꑭ Chebyrator ꑭ, Seeker_FR, sladjan_boi, Lined04, jacel1213, ClaymatorYT, BERSEK, GOLDEN MOON, Memez, babyjuncalma, zi2htb, Illya_08, антон-андроид2, DARYLLY►☻♦, Jakubek, Jakubek, Shoyo>Mia, segurahanley, troll back in 2061, Senko San, Axel, masuo ożehowe, idhidden110, Goose, А калтофка есть?, liamraghav13, [PRIME]_ПРОРОК_, VEHICLEGAMES, TF2 soldier, TF2 soldier, Jason_Voorhees5351616, ......, Kulgoodmxo, Er, Killer Crab, PowOwu, fockerer.family, Noir [PL], Noir [PL], 다크히포, liwidum, bill, Henry, Majlo, yemafa9507, Suprise!, pilot778 +777 others