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.
Frieman, A-l-e-k-s-e-y, Усама, Gustav, VALVO "MŞA", Vilciņš, 〈VIPER PLAY〉[ViperRP], Greedy, ERROR!Sans, GreeN, Yura Borisevich, Lavy, alejomolina, Die_Echte_Lisa, Jarkom, Хипер, Performence, SkilledNinja csgoboss.com, VIP_JS_VIP, bOnK!, PasSanger to HELL, SolEK, QAWS3D, Swaffelaar, J Е W I S Н А L I Е N, XYILA, ReactLP, @БРАТИШКА@, ✘ Erebus, {BHC} Rec. Captain Bombastic, KKK, JaytheCat8, ghost.pro, ScopZ, SpuckiTV, Mr Soarin, trenagekzam, dddlcaey, CSGOBIG Bot #04, youtube.com/c/ExdisionCSGO, Besoffene Tomate, fudo.su, MustafaHD[TR] Ressy, Princess Celestia, meoooooow!, eXquiisTe, nifares, Dared_Evil, Hantoren, mitzel +66 others