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.
dead.undude, Wheatentail, jrobrox, ✪ Marcus -M-, KJKJLK, yebraa966, DeadPixelz01, Christian, joawnai, ▼-=Foxtrot=-, ▼ basicdoge, [HellForge]Visto, noobplayer151, Liam | WickerMan, Vеllу, Sistine, MegaLodon45, sinka1031