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.
Supreme for life™, maksonmani, maksonmani, egorkhramtsov925, houdaonly_123, Budgiez, nekono_w, Tracy, bortonator, GJ19964, unclebody, Isaac Hipkiss, tssthesplitscreen, Rubisetcie, 3D Waffle, gianqueen, Shiagur, cristiany1596, The|Percy™, Robert Throbert, Nafri Zero, Wolfish EL CUCARACHIN, =PP= 419