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.
andresdeleonpaniagua1997, That By fley, Savagely Random, JulianPlay, sawadas9999, yisuz70, Spyroxxl, CHEAZER, sebastianestrelladiaz, ilia, Best Tofu NA, Toa-CALDEX, The JMP, Fonndor, [TFN] Warrior, Bulanda, diane, VEXYTOR, The king of cinders, Overlord Bubbles, Ben, nakkalokesh966, JustinDayQ, Yung Dave, Wheat-Os, Agent Bott, rgunn4, 00miah, RoZeToChKa#, Critical Tortoise, jrickaby2002, xiongliangtian123, dandanabique, Porumbelul, ♫ Wub Master J ♫, DerrickScottII, We_still-in-TH, p50446.2015, ☾Moonman221998☽, andree.aichi, andree.aichi, ysraelmoreno, Ayy Lmao, yan.zotoff, cryptidhuman, Majonese, Majonese, Yadio |, Yadio |, Big Box +261 others