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.
a0955282209, Justiq Sx, RxZ_MaxLeFou95, tommel030, bossanac99, Mr.Daddy, rmderoode, JAN.HYMR, Shaq, Kim Jong Un, jgvg24, dofasquel, ..., Trager, Nighti :D, pera9817, Alpharo0007, betlegend.na, Dragon112322, RedxBull, RedxBull, ByPoLaT [TR]