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.
Jaws, Strelok, [DCE]Ipay CSGO500, alfredo.garabito2002, A55A551N0, enesmustafabnBN, andresdeleonpaniagua1997, RngrDeath, lilidi.castro, muhamadalfikri_pesawat, Kaskobi, toniskylet, CaptainCraftin, Keeves