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.
\Frix/ hellstore.net, cartellowrider, csgofast.com™TeamRLX.G2A.Ке, ุR, glenngregson1, doom3dude, superbro, General  Plastan [RuNovo], -=Vethqype |CMBFan|=-, alashi.mosterskull.monsterskull, archicastor1, amirardeshir, Slavic Pigeon, [TBP}Grapnar, LimeWire314, LimeWire314, spaghattosnoodles97, faizamfomegnam, zaza123, wolfganx, Balkan Guy, octboss2003, SERJAUMHUNTER, Clever Name Here, pinesalexis, Arexa, Wicked, robbed by a chicken, robbed by a chicken, robbed by a chicken, Chomper, Chomper, Chomper, Maamsou.93 , Sliver, Max Maclayne, megadarkotaku, gilgacat, UncleSHREDDER, ::[email protected] [email protected]::, Malone.j2, adapio040, dermo v time!!!!!!!!, [WNM]Her01n1, David Allan, David Allan, Пожилая Скумбрия, иоcтёdоцт, иоcтёdоцт, |Messi is The Best of All Time| +1 others