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.
mossesx, antonidragonesgo, Sans_ewe, SbireRocket, CeasarWolf, Brisbane—Destructo, maks_peyn_t, IIFRZII, tariqsw69, KingKryptic, pacino80, tallierik, <..Kacper02PL...>, ConsKrypt, Alan, Saaabiiiiite, nonamethe2, resistor58, resistor58, dat rushen boi, ♥FinnTheCuteUmbreon♥, Shigure Shiratsuyu| 時雨改二, BLaMotte