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.
tasostrig2, Vestige Vapor, ˋ·ㄥÆㄒ·ˊ, fk134679852, The_Childish_Psychopath, bruno_millona2010, bruno_millona2010, Templario Cazador de Marranos, Ømiser, Kanye West, matheosaussaye0901, Yuren Dae, Deme「Sejha」, wekaviz, sharkman-11, Proredstone20YT, MK Kahnivore, Shadow, pawelmichalak005, lisandro226, lucasespindola809, gunersimal, Dr. Beetlejuice, Paragon | Hawkwood, erolwolf072, freekill361, DiZiO5, amirul.amran290, sauce extreme, tomascano.romani, isaaclevyt21, Bradesco, OlaDuWa, Sailent, illegλlshot..., andresfrergut, Rokimin, The-Master-Mind, mossesx, Gato2, ✪Foix_, AGoJyt, Hammer World Editor, grayfamilytn, Fyshi, abz_cool66, じょえもん, coachs20003, Robotecskullito, Radical Revy-chan, [TWHFC] 'pond' +116 others