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.
Neondromeda, frdhog, Ⓝecroи99, 9 night, zanofa88, DrMystic, Damlinor, Marvelek13, thasquidkid, angeldo, [DreamBuilder], 7`U80, Kraken, Balou19981, XxnabosxX, MadcatMK6, thatmotorfreak, cypres master, cypres master, cypres master, tarper24, julyboy, Yassel22, †H△† Be▲Ʀ B△5†aƦD, Chuck! No, Captain Chuck!, Alex, Cyber Ninja, TOM FROM TOMMYLAND :D, thepipefox, Anson, Helios Dik