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.
marsh, Yordanovvv, Marco22, Barkolas_TR, ABSD546316187, noodlemop, stevenjared2016, S, [πΠπ] greengeco, liamhorsley10, [ Zoo Nim Drop The Bit ], sampistola, 『AVA』Ris, themadshark44, 이훈, Trystonc, Kraken, qxtat1234, quinten.kustermans, [k]chy8904, Juggernaut92, daisyhatestacos, Human3069, G-MON, JoJo, zchloopy, Randy Orton, Their watching us, NoobMagik30, Dustbunny, 629th Assault Artillery Division, c00pert, MCLORD, GodisDead, =PP= 419