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.
yamenturk2010, AxmyvHD, PapagáJ ツ, PapagáJ ツ, rahmany099, 27alaing, Ayranmen, thegamecrackzzz, nETPOBu4, ( ͡° ͜ʖ ͡°）, ( ͡° ͜ʖ ͡°）, Kartoffelsalat, koroglualperen014, Dick the Balls, Alois Transy, kgergo5800, 674pain, Danjochek, |-ICE-|thatonethingpersonboy, purpleman586, Petyx, Hickya, andresdeleonpaniagua1997, death by salt, bob the duck, [ZTF] Unoriginal, BigRig wuz here, Ｋａｙｘｙ, BlonkoGoesBOOM, ⏚, T o T, PsyKoTMK, TheRandomPlayer, DecadeFury567.exe, VisorMachine™, Dindex, GUNMAN280, truskiek1, CarlmanZ, Hank Kits, Martin DR, JrLemonsHD, Toroko has a gun, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, Reverend V92, A Deep Commitment To Konata, Napas +10 others