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.
[Wolf], marcusd93johnson, manueldejesus57433, Venryion, Dovahkiin, kuripo91, bane0333, gostya66678, lavacakes28, ready0die, GRIZYUN, ＨＡＤＥＳ, moonglow1970, Harambe Uchiha, 15977989225, Oe Wea, vladisav.simonow, nigel92356, -x[M]b^ -=Apollo=-, maulorxd, Vinziiiz | Twitch.tv/vinziiiz,