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.
VcTeF, take0714, J.Kimchiman, otsumami1999, zxc123456789, nurbettymaria2607, asai_kmh, Masaru, [JP]Nekozeryu, haruharu46463, edvyyys, hidebu12, DnM.MrAlfredo, koonmos ch, katuo000000404, bigoyo, Drako715, muruhu6814, TerapinBu, Noah_SW, Fjjm, Hds46, CheezMan, ikosmid