This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Th1m0, [AUT] DomiP/Dominic, [Katana]PrincessFluffybutt, lenny007, [GHZ] Rene/Severus/Sirius/Johnny, POTATO69, A†ri, Bravo2YTpl, Slay, MisterAlpha, ATopHat101, Pomerl, Zeraslight, Knirykas, xLiQuiD, The_Hunter, DarkCitizen, TK, Kartoffed, [SpJu]Bombyy, Kuddelmuh, Apo, Diamond, KeDo, SuchJustinn, Reddifusion, Denys, Inside Job, Blue Bonnie The Bunny, Eterna, balazs5001, chrish5168, George, ChooMcoo, Brisbane—Destructo, Deadisland11, 10081971z, Kaos, OV Red Storm, nizamazrol7, cron.lol, Colonel Krud, stibennikelson66, [tooNoObs] NjesOfCourse, G30An08di01, Crazy Jay, Abbyson, blackhand122, MasterJPixel, stupidforce +435 others