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.
ღAngelAD96シ, $Zippo$, Ashie, tygrrus555, edgarsgrivins, Dark Magician, frolov.a.m1, Smol_Nana, ◈Ḿ, Stilgar, hhhh666, m4rc14n1t017, kak.oleg, [G.Z.O.S] Stalin Mobile, Confirmations, Ektorp, Blixer, Harambe Uchiha, itakacs72, CorvoMx, fakema, mossesx, exfirebr5, 404ProfileNotFound, 404ProfileNotFound, Falgrom, -x[M]b^ -=Apollo=-, BrendanE, thomasrodriguezcabj, dylan.hill2272, Geo, dasdaag, maxmclain1212, Serzhsokolov80, JM, Anyone999991, kangur845, kangur845, aldohn608, enzopoto, Smootis, Fe35e, ThenewFriday, steveotto999, XDLOG 13 [FR], mertinci1907, geny, JackalWinter, OneStep, Drifter108 +114 others