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.
frivan, gabito.gonzalez.2003, Walker L-N.GG, DJubbert, Polonius, λ7Prime, Bekλh, lil honda civic, Jackround12353, berryaaron11, spit bitch, Brisbane—Destructo, hart, bennet, Crazy31139, BenFuller08032000, Dusk, Szaifedias, AgroCreeper37, MasterLord140, Anony, Ortoblast, gomibius, DavidTigerfang, elarturo_96, ismaelmaster3000, jjsloniger, wet, resistor58, Barry Burton, Ҳτгεӎε⁞Queeenchies! ou o, おっぱい, Loki, KyoDulac, Starstilanx/Verinax, Lead, TheGavinator, Howl, SealGunman, Dune Jumper