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.
Classy The Third Shnassy, Phil Wilcox, Jr., tiptopspin, re15revisited, re15revisited, Spartykins, onelinedisco, Heavy weapons guy, Rammy The Edgy Lammy, khushsikka7, Kennedy., heater, rmn1981, raymond1922, TeleScript, BOT Doge-Dog /S/, mondoman, woof, [HG] NightShade, Icetee412, Scott, demonsoul, Jo291, elfrikidecaballito, Dark_shadow24, S3Killua, [NC] Dindex, HALFLIF3THR33, ETPA Prince Tavish Praey, l.maire c, Jeff Dunham; Relevant Comedian, RIP AND TEAR, Warpath-the-drone swe, Kylharc The Sinistar, Icewine Rose, NEO_EX, MyCbEH, Jei-Sun, mishaizgrezino, Sidman, BronyVNCZ, Sirgibsalot, Papasifis, andrewdbuilder, andrewdbuilder