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.
[MG]FOR EVER, Mr. Merte, Randy, Gecko™, SKW| Siv, zanofa88, ΛП†O, TryHardGamer115, Shnawly, Panferox, Nick Neger, Frank, DaGloaDone, BlackB0X, leon, Grantelbart, heinricke11, no.za, Robert Jonson, Luffy, Razzation, Onwer, MariuS, R0XX0Я, DerEinsameTOD, BG|FreeZe, NiZeOne, Cr3nk_player, Vosssy, Anthony, Tr3u3r, Sic_Coffeepuppy, acool_berlin, Definetly not Apple Pen, CarriedByKevin, Ro-Robin, re15revisited, Tonko, MaPa99, Astaroth, Jenkee Reemer, GG Keyrem, DingDongTP, Mönch Panger, TheRiddler, Rabbit Touka, Thé Tai, romkalenivec228336, TOO-BEE!!, Windwhistle +60 others