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.
[G.E.W.P] G-unit, bashir, darkstar1168, John Marchant, Captain Harlock, spit bitch, Thai (โฟกัส), Green Bert, LastZlo, LokaMas, LokaMas, re15revisited, [RP901]_Randy_Sparks, KittyCatMeowMeow, PinkFloof, SaltySailor.Fuzzy, raymond1922, Denis (Governo Nerd), axel.gembe, PCb jacyte, doreply65, doreply65, doreply65, Scott, muho_-255, ♤Isaac Clarke, michaeljamescalkins, Clutch2265, HALFLIF3THR33, Ulala, คนไทยรักชา, Svensis, Damagelink (DET Co-owner), Galax, gloowb, cassiokid10, Tazmily, Lord Eoss, BLλZING_DUST, Kaathe, Roby, DyMare, It's macabre!, park smoke, [BA] Brucektrain (DEAF), 11mHz, Teddi, thirsha4242, SCP-079, Howl +4 others