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.
Alexandre, mrtrixiz, canalmultiverso, kbubuka, redmanwalking1, redmanwalking1, VendettaGamer11_YT, MARTIN SPONGEBOB, Bamm Bamm, [✯R♚L✯] Mike Dayne, jofuck_yedka, pro125gamerbg, busermanov5, Sr. Alface, Sr. Alface, M_G_07, parro192, GodisDead, =PP= 419, Extremskilllzz893, BillyGoat