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:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

Salame, o(>ω<)o✘ОЧႸ В ՈИТЕP, BIGROBizzle, Key-Drop.pl Hvk <3OLIWIA, Segundo, [GL] Robby Peppers, mr4410192, johninchesirdu76200, elbo1152, Julien Miller CityRP Gaming, xfrogygamerx, P.aXXI09, OSCAR RP tro2fake, Mrskillz24, tatatanato, iToady, canadianninja4, jdog11003, Michel Tenzi, GadoZOfficial, MeGaDevice, roites666, youngethan888, Rabbit Krosh, RaidGame200, WhiteSnake, Papa, ɅZɅNɅ, Tobi[AzeriaRP], ✪ DrUnKeD-MaN SK ✪, humphreyg0999, temur_p, [CGN] Cody, wolfnod, xWailyn, Zack, x_THOR_x, Maru Maru ><, jameshj84, RAINBOW CRASH, TntN_Ram, KeWiiPL, Voz De Ouro, dh13080, sanozingsan1, penguinXXXeater, DaSmoothGamer, AlwaysDarkLife, IballistiCollin, Joseph B. +1029 others