This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
re15revisited, __@ate, Renchary, germany, Dylpickle, LaFolleDu03, *m8 Rekt You, [MiS] Bazoom, Burning Bandit, Shimothy, KuronekoSenpai, Epssii ¯\_(ツ)_/¯ Dylan *, JacobIsG, I am a Zombie, ImHADI_, Vankate, Nightwolf, OptiWooty, PsychoticPanda, TINKY WINKY ♫, SabreTeam1, SabreTeam2, Cody Collins, ☢DukeNukemBOS☢, ✪Sheep✪, Connor™, Redfox, KILLPADOG, ohadba, Solvanos, [SG|A]SkillzZvStaR, SadGuy, zero974, x_pokemon, [SG|A]SkillzZvDaZzLe, hiagoquiroga, barrelbiscuit, ~ALG~Xanxus, Mature Gamers Network, Jose Cuervo, yung dab, Tommy has a Panic Attack!!, NawNaw, =RGS= Mr.ROFLMAN™, =PP= 419, [GS] Kriz, cs go sucks lol cod is better, don_sulten, RazQ, X dude Awesome +1 others