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.
Saysay, GalalingGamerTv, grucha19869, grucha19869, grucha19869, tindail, AnkOoz, rafaelsilveira2002, ĹĨŢ†Ꮭ£ ᎯṦ$ℍØℒĖ, mghtyjester22, cwilliamjay, Mbah_N4k4l, axelmedina633, Ra Sull Gull, Zmioru, DATMADMOOSE, jesusaiyan, Bukva_123_i_znak, victor_motta-, Sokkah Abdelkader