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.
xandr`, baiermichael01, jason voorhees, aegisblessed, bmxtriker, lukamedolishvili7, nicitaofficial5, Erdogan_kicked_Russians_Ass, roekiyah18, Demon201666, mossesx, muyucat6, usman.edelkhanov43, theo.pons, arianasheghi2377, miiyu079, ✔ℇki ッ, OBI WAN, ♠Hell∞Boy♠, ramosleonardo833, Mr. Smail, malfar212, ahmud8806, Francis'DeLaVega, I BARELY EVEN, supqttqjaithqmdee18, shackle_playz, Vanoss076, Tostinos Pizza Rolls, nakpk42, pp08250, vitinho.hugo1890, leon s kennedy, carolzinha_pereira14, Татарuн XS PR0, Jfldreadnoughts, AGKK Histoire, thailandtin2548, marcs2017, King Fluffybuns, DJ Octavio, General Luck, General Luck, kurohana12, suoidiS, dhruvpatel0713, Âbruptly, ◢ k r i s ◤, lucasanimallover, BruhSay +996 others