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.
facundo.romano18, andresdeleonpaniagua1997, darkplaygames, 1172936548, muhamadalfikri_pesawat, muhamadalfikri_pesawat, seifusama, XticGaming, _Man., wonk, ridhaapetersen, Porter Justice, aalik1066, aalik1066, WI A ELECTRIK)))))), LolerGR, StegoΣ, Ҳτгεӎε⁞Queeenchies! ou o, OH YEAH SWAG IN MAH NIPS, Rawr, squeeze, Sokkah Abdelkader