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.
swat, Fruits847, ivan4290, dinhgiakiet23092007, [KOR]붐스틱, SneaX, [CKG] Epsilon [HA], Kirill, Diabolo, Diabolo, Jun0 CS.MONEY, 苏联亨氏病 >.<☭, 苏联亨氏病 >.<☭, The Immitator, UnderstoodDerrick, anomalous033, anomalous033, smurf(not real2), Julsvor, BIGVROOM, Sov, Duolingo Bird, Wierthy, slightprinciple, JaK, Tudor HD, mysterydude03, Gary, C-687, [TN] JackJack [C.E.O.], [TN] JackJack [C.E.O.], Johny Cock, Misty, Snow_wolf31, demoman from minecraft, SammE, SammE, Rambo, kermitstalker, [IFN] Mikey [HA], tkacenko1958, shadowstalker, hunterdeer09878, Feyeve, kira2008228, dobre shabowe, #fake, Pink|✘෴✘|Simulator, Megapihar, [GL]Tator Tots before Thots™ +478 others
