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.
♡Festive Becky SnowFang™, Lazysocks, ral321, littlezack, jto10, Teh Awesome Epic Face, gabrielgantz87, I Know Right! #Mother4, HyperKeep', Mickey, Adam * connect darkrp.ggdu.de, MiLES, Asome3333e1 | Trade.tf, Upraski, DAKHARGamer, loupgarou_e666, Lefty Wilbury, lonely19682005, Radastage, Drunken Luna, if @School then return "Kys" end, DvD