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.
Truth, leoa690, Holden Makok, Smitty Werbenjagermenjensen, PretZalz, I will show my boobs for £20, SheaShea0524, ✪Uway.ﮐ, Pickullz, ASAP BROCKY, xKanyeDark, Devigorous, Esper, OMGitsGamingBlade, Slinky, ÜberPony.exe, innin, Here In My Garage uh...., BillNyeTheRussianSpy, Anonymous, ^tPapa ^tAcachalla [PAA], Tomas Marlen, Timmi6790, Queen Elsa of Arendelle, It wasn't me, Doktor haus, Marauderx23, TheWalkingToast ♥, DJ Doge, MadScientistPotato, MathieuDolovan, RadioactiveKen, NyarkoO, Caleon1, Hyper_Foxy, Rhoken, 3dgy_Santa_Boi, 3dgy_Santa_Boi, badian76