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.
re15revisited, alexseyyyy, wailik2003, unibits02, danilonerd100, Mondo❤, RASTAKITTEN, rongta, Korbis, OneTime, V_O_J_V_O_D_A, God Of God, UncleSHREDDER, Histerie&Skify | kickback.com, Watchdog, Doktor haus, Victorious, Victorious, Vodka, Black Knight, Jojo Manning, NecroPro18, Warpath-the-drone swe, Farfanugan, maximoran1996, Cashkingb, eylamshvarz2, 3dgy_Santa_Boi, Twilight Sparkle, jdramirezelmo, Natural Selection, ikosmid, тномаs нunтеr, Buzzofwar