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.
Romsbord, #PayForHarambe | Yadon, therainbowfox1, -|KNG|- 애플파이, rifqi.syabani, gabimagloviti, vicenteav2002, adenildacsme, alessandro02c, Matt, hugounivers6 rm, tothandraas03, mfut00000, orcclans, wachis.17, Mystery man, MorgothZEone, brenkusak, xXFurryLoverXx [ITA], triptychlca, Scorcher, Riha.Jarous, sanchezmarcos702, TheRman146, sonicteam998, Masterspy, ismael1878, Sup3rNo7a, Street - SERVERS UP @ 16:00 BST, Nyoom Nyoom Nugget, TheMaster, DANJ16™, peytony9, The Insane Hunter, Hooby, Tal Kopel