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.
Niklaus Mikaelson, muhrezki4, Lucy"Melody"Addams, Lucy"Melody"Addams, ilmingostyle, Sora, KarmaArtz, [HMIB_Guard_SSgt] Calvin, SalvatoreMasterDenif, TheFreeman202, haydon546, Festive Lucina, Festive Lucina, Ryuunosuke_FK, Swat Max, Aquila Ursus, kutulu.kutu, EwanBlack2000, Neicks, Satan, Alexandre, WolfDampf, ytp123, ytp123, Peredos, Carrot^, Agent Hive, Mike, drumstick1113, Sup3rNo7a, Warpath-the-drone swe, ACR_Hutch, maximoran1996, lonely19682005, do_i_ob, TetsuJin, JDRider, jdramirezelmo