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.
Versin, princemurf77, yamilbordon175, landonbutimfat, Index, [CN]QBZ-95, SickSpiderYoGaming, pray47039, WitsEndGamingOfficial, GoldCro, 123, SpringsTea, <#F00>Smyclone, -_Dolmrus_-, Moshe, twitch.tv/allz0r, botchmarcus, Don 3, я не B1t, LEGEND., arihas20, Lolo, Dyszz, Auvernity, unga, [SG] Asuky, demirtas-sam, 萨沙玩, ruslanzaw, Krobalt, Kerbo [Main], Cainiven, Hadge, somjai02563, MrCreativeLP, Mia, SaladBoi37, flavien.zebzda, gamekiller 3.0, gamekiller 3.0, gamekiller 3.0, Try_Crafter, TheMagicKing2009, , _YellowSky_, Vea Time, Nightmare, Endersim [GER] [MG], Endersim [GER] [MG], Space Cat™ +2878 others