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.
yackson, Lahtineer, cdktiger /\ cases2x.com, Drexyl, dipanjanc33, Joker, barankaplan05, The Praetorian General, British Imam Ranger, LOYALCHILD, Fox 666, P.O.D BOT, bossbreaker45, BeatDR0P, Greenfire, trickyman1337, Nicole-Coleslaw, WaiKuang, Zakkybongo, victor_9212, Extraction, Lach Anonym, TERKA G, godzillamonkey, DafoeCan, Cam & lewis, PraiseLordGaben, DarkenWafflen, EzioJosh, Chica, domozero7, Y_R_H, To_The_Max, Y'know, the one who died 5 times, mickeymouze5, d3rpman, Mr. Cheekyface, DaWhale, GT601, bmizzi08, slapper078, STOPHER, 3tpdʇƐ, fartsmellovly, [DFS] ZecGames400, ♠BoS♠ tikva, Darkteen, jaredcohen_8, Toy Chica, UNImapp +14 others