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.
Ghost, [email protected], NiZeOne, Bloster99, Sazuro | German Wolf | KC, fightherot, re15revisited, re15revisited, Brisbane—Destructo, mazedummy, Tobster57, Bunneh, Voltagepyro, Enot21, Ack Ack, bautista13angelica, erimezmert3, salman.hafizh, Wishmaster, rmn1981, Xeno, dmitriy422, dvinauriya, shaqnd32, Studge, Chris, Windwhistle, DJ Memory, Master of Death, [JUKE] Longneck, viperosu74, judge judy, __Nesquik, Bowen, [GG] GucciCarry, Rimander, brancomeister, Chilybones, xsupremusiepiex, Monkeynaut., [TWD]Prinzessin e4sybella, LPG, FreakinZombie, Kiwi, Join Nineteen's Funtime in TTT!, [KOR]혼살InLv, UncleSHREDDER, Mooks | Clearing friendlist, dizla, Ironsufex +30 others