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.
2ET5UB0U~5AN, Pikadactol, 『 』, xXKillZoneXx, koongolf009, Storm0325 #scared #pingattack, lukewheable7, Anonym16hack, peterhuy, greenhood, Schwarz, Dead, pac man, Haruna, Ethan Jameson, Cody1138, sshahrabadi92, xHaRdyY.Z'@GamageR.com, zarbon5000, danaraprapanca, Egoshooter, JMT, Luscious_Blobfish, Riku, JenEmMe, RudyX, Scooter, krycha653, OvErlordHQ, marchante_23, joferson1320, hogr3, Adminwarrior, ζeh Matt, nevik609, timizorzom, ︻ヂPunish☠pooL◤══一, milan7943, 3354334790, ssh99o927, VPS.Malcomn, VCayres, Vex, Caribbean Jesus, DBurner9, Chrismack648, 9/11 did bush, saba.beroshvili, Lee.Purdy3, metroguy312 +109 others