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.
Squeeze, A641, T o T, G10, G10, danil.ch2012, Lordman Gaming, AmazeGamer, lucas56bkn, 휴지, Anonymous, sebasr_96, daniel.e.crissey, Coincidental, BloodierWrex, pacman107, Ivanhoe20, harrison.ducommun, Grozger Zasrogtiveslin, maykol522, Laguna, chlrud951, Ivan02, BigWAT, JoshDBoss, ViperPoison, DesertHare, smb123w64gb, outgoing4325, Karlops the pancakelord, Karlops the pancakelord, mkloiyiu133, Blazingfire, hectortito612, nerdy_fool101, simon11, KingShadow, Master_Croc, Deika, JeremyVannes[UK], JeremyVannes[UK], Sir_browness, Micheal?, LazerSpy, LazerSpy, KamaZ