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.
theo_lemoine, henri.jan.kerguistel, CarpenterCat, afiqlukman69, [KiLL_DeBiLL], [GEWP] Killbow, godlyknight84, xXMetalCastXx, Viktor, Runatikku deddo musuko, Nico, Bucky, [NWA] Derpasaurus, Fatal Exception, Abadu Gaben, Penguinson, iheartscott15, Alpheas, lankyman10, -=ATI=-, MichalK, Lys Furansu [FRA], FentonAlpha, I Kill You In My Dream, Bluehawk1224, Robert_DaRkNeS, sebastiencastagnet, LIFETAKER_2276, Fluffy, crackk110, Echo the Bat pony, Goku Black, jgvg24, Golden12, johann.barn, （C）, Chibo, Iced Coffee, Iced Coffee, killstreak, DJricsi