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.
hasdaa, (RFAI) Sugger, stevenhughes38, Nao, ✨ StarSparkleNova ✨, Wojtek, Hackcraft, MirrorReach, Vandarxx, %%, John Pykett, Agent Agrimar, csrss, Sokkah Abdelkader, MarioDox, ԶιяєΧ, ԶιяєΧ, ԶιяєΧ, ԶιяєΧ, Spastik