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.
03kj20f03232, ♕ Jσɾԃαɳ [VLRP] ♕, Destiny, Gurev, Redstarwolf, re15revisited, KingKryptic, timmy999timmy, minionguy, vL wolF, ☢☣Takumori☣☢, LightsoffGaming, hydenbully34, Ethinity, El-Toro, db4ks, wet, Bill Clinton The Rapist, MangoJuice , Diggingwolf2605, Zackyo, spartanairsoft99™, sannu9, Hit, ☜UniiQuee☞ CSGOFunPot.com