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.
robotmaster2006, alexshulski, re15revisited, toma_drive, xXSpector164Xx, vik_a47, sukoisu47, Rizal Borisman, Morehouse928, Wolfman19682005, mistblood, JimTheCookie, [BB2] Faith, saba, muaz615, ikosmid