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.
The_Dragon578, gamergameplays14juegos, REAPER!, DarkAvengerFON, Koloax, ngernzaza, -[FF]-shadic89, DeViNee, Dota 2 Gamer, iulian_2012, iulian_2012, realdantdm375, lelargetheo, plr5, xidandrei, < Lucas >, 【哈吧狗】 伊森, Slava, Goldfreak