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.
DarkRiser, joasiazak, antonietta1983, rlz, martingarage, kingofcats95, pppprrrraaaaaaaa, PvPizza, btorgerson2009, piplup_the_panda, TheBeST, gabrielsousagomes3, iulian_2012, jose93, kan276310, falseprophet141, desinteresado124, QTG坑, nanmu122, krysti123, ♛HHS, SamuraiSven27, BubbleFuck, Joey ヽ(⩾Ѡ⩽)ﾉ, FuzzyLiama, TheHeroDeserved, ♥ Princess Cherry ♥, Jantam11, ☢Adeft☢ FiestaZ, potok32, Agent Hive, nightmares, jdhr24, an.krawczyk, How Super Terrible?, How Super Terrible?, How Super Terrible?, ☭╬☠Killer2013 Rock.003[TH]☠♥, MLGgamer