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.
Shockinator608, 03kj20f03232, Evil_Aka_Witcher, Unundying, Vicror Tsoy Forever, KIR, YT_Lev1tate, mossesx, Luke | USNSWC, Sheep Wizard <3, OG9, INsane [dodbits], brynparker2017, Souldia, Soranyan, Wolfy268, ＤＵＳＫ, VVTT, Patch, CeddySpectator, mnami690, Second, NilssMiner99, LOyoujoLI, jean.ba66820, StormyTerror39, [BG] Hody, XAZOR, Bob, Goharjavaid8, Aber31385, Emilio_Bus, FaZeCu/\/\Sh0t, Koneko Toujou, monteins., Waqa, victor_9212, Luke smackdvraw Nukem, Feier68, crester, ConsKrypt, ConsKrypt, | The-G4M3R |, SKINMAKER, Pekingente, DENUGAN, VG | Jason Voorhees [GER], Bustinuts, Bluewis, Ogyr4uk PIDORHAX +37 others