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.
parkergruber1, Я ЗЛОЙ МРАК! ^_^, mossesx, demon nv, scout, knownewsletter, SAS Zombie, moh_201, shhed1111, Korkunclu Soyguncu, Dual Clash, Dual Clash, Blue Bonnie The Bunny, Costi Stoica, gabriel, ᕮ ◥▶Revolver◀◤ ᕭ, s6539966, RedHoovy, enzopell, LIKE A BOSS !, <:::::Fd GoOd 14:::::>, spongebobf19, kepler3000, hileliadam-13, ❖ MAX ❖ I ❤Toriel, MLG Shrek, senka02, senka02, Goat, orcclans, The Trooper(FIN), samashpalm, David, the Coffee Craver, Lord TL, Legoben2, El pelon Blaco, XtremeTerminator4, XtremeTerminator4, DecoyofChivalry, Santanist, George 08, Zokrau, LazerSpy, Soviet Comrade