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.
mati12345 lolo, Kagami^-^, error 64, valamilyenérzésHUN, skdnhygadtf, 8259026196, czarneckiwojtek6, eduardobautista254, miloszkruczkowski9, stas.g, berkhantokdem32, fififilip24, Art1515, chefjonaschefroblox, ryanmikolajczuk KeyDrop.com, goldzikps, 159pansosna159, Gugu_ProGaming, tatewiczrr, NinjaDec, Fridix, penispierdzibak, begich12, Nesko9 KEYDROP, PanJabucho, kaczuszka.ty, martinezromeroleon, Ivanekk, konrad978009, Daniolek, NZ_bartek, Jugg3r, Dragonlarte, fabianosik13, Swioteczny piesel, MiuST, adas.paczkowski, ItzzGuziec, jddv15d, 6.3 SIBEN, gamerkeke219, wids, damianwolpiuk263, v.i.p, saloom.m.hero, haniabonifacy, Julimiro, LaynusST, muhamedalibaba69, MangleXrey_YT +4540 others