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.
gasmasked999, Tiger_Soul89, herobg1234567, Volodya Marchenko, Volodya Marchenko, SonGokukid, toygu42, hassan.lattakiaa, zigortfurkan, horsinator, mario.meme.exe, Player, Eliot07, EinsZweiDrei, mossesx, ShaunDash, ewadan255, alexshulski, unclebody, danilstarbird, bonnie.opazo41, M.granis, re15revisited, re15revisited, DDDCBP, heatsaber, Sire407, MrAz18485, serkan_guler18, assasin123, | モリタカ |, dmitriy-s-98, Russia Greatest Love Machine, 1ssue, KelluTxdd, UncleSHREDDER, Ｊｉｒｒｏ, Alexis, goldfishburgers, BenZWinter, Jack, KHXhero, thirsha4242, andrewdbuilder