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.
remote.televison, OrestsNx, zekeamongus, Kasta, attlit, Scottish Summer, DLLfile, penis, SimonPWNer26, I really don't know, alfredodestructor10, relle263, f2p gaming, Subway shooter, Coppermind, GasMaskCitizen, GasMaskCitizen, Bot_1, Skull_Hn, ale3036, SleepyDino, mrgarita, SeuljiChan, Rabbot, flameguild.discord, mbazzi359, BurN!!P0TIS!!, Y3g0r_P0rtalov, l3onav4s, Dzola, fabritogames5, chochin7675, spy 帥哥-SPY handsome, MalwareDud3, alanziz13895, themanhacc, themanhacc, Shichin, <<SHADOWGUN | DEADZONE>>, Wontairr, ุR, The Master Hack, Don Miguel, Don Miguel, juandiegorex57, Nexus gaming, Nexus gaming, fetato122, iToady, Tyrannotitan Fan +115 others