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.
re15revisited, re15revisited, Xibo mata, Noire, Ackua, rmn1981, Rhode, DarkKatex, DeXe36, TonyTone, arsenicCatnip, Shadow, ไอเจ้าเน็ส, Ninja [FIN], JustAddRainbowDash, ☜═㋡ ƊɑϮ ℒαɗ ㋡═☞, Goose, Archer [HPMC], Scythane, meh, Infinity, [UwG] Canvenante, The Mad Hatter, Doomedpanda, Baron, Cassius Faux, MessiahDEG, Jack5868, Shizu, the Silenced One, Bluedragon (Talon), [RPC:HC] CMR99, mygotic, conraad, DanDaMan, ♥♫♥ KazeNoTora ♥♫♥, Real Madrid, raygun gothic, Warren, Apples, ThatDutchGuy™