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.
jordenramshorst, Sam the Man, BokMustang, kgergo5800, dumitrualex119, starraptore, jesus_sanlopez, burnouth03, kangur845, jond0000, zefriss, _f., CantFindBacon, therayninja, nerdsplay brasil oficial, jurassisgamer, Sigma, wwwww369, DoggoficX90 - DAT BOI, flyer22012, man, gaming55.teo, re15revisited, custom.person, custom.person, BOURGIESTofBEATEMUPS, Ross, Squeeze, Joe, David, Грибабас, Emerald Pixel Gaming, Futureshh, DRxSoulz, Shadow l KryptonNetworks, Manchester 25, lellicarlo, 1035821587, Gretto, Impulsive.i., [SNS] Xwatch1009, LuketheBrain, Cory B, [Oz] Proto The Gothic Prince, [Liška] LadaBoxCZ, smbxmcgm, Cloudy15, AngelImperfeito, victor77727, ironknob2000 +39 others
