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.
supercreper111, Uragiri, undeadsoldier0112, Flamer, decadent_uk100, My Life is Complete Without You, sunset, Grozger Zasrogtiveslin, :D, Yuri Petrenko, TortaNaKaprovy, Isaac_clarke90, ФЕВРУЛАЙ, (TR)Bozkurt, (TR)Bozkurt, ThrasherLew, The Mysterious Gentleman, tikva, tikva, tikva, tikva, tikva, tikva, Hell-met, Lhyx, Big DJ Linda (DreadOut), L4D2Fan13 (Birthday in 5 Days), L4D2Fan13 (Birthday in 5 Days), L4D2Fan13 (Birthday in 5 Days), L4D2Fan13 (Birthday in 5 Days), Forg(R)otten, Jeff Dunham; Relevant Comedian, Winxomatique, Warpath-the-drone swe, Warpath-the-drone swe, lonely19682005, Sir_browness, GojiBob, KotenokGAV, SoulSentinel, Birdman, Birdman, minebuild02, ◢◤Sixten◢◤