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.
Jankes567, Heckelex, ACE, G-MON, Alfred Jodl, RedLegionnaire3u68, WH|b00mzik, Chloe, Kevin Pule, huymaicong, 피트, ole to tu polla #EPK, TheHeavyIsASpy, leventebago0602(HUN), wangshiping3507, HoodNigga, yankers, Lucien de Patoukian, nguyenphucthinh1998, damian.nowis, FiReBoSS, -[TDF]-TEMPLIER-22, Ramon2311, DrakeAcheron, Trauma, boblealeonard, polllalala