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.
wiolletta20133, spaghattosnoodles97, re15revisited, b0ssANtonIo97B:D, alexis.muller, Tel-Ash The Juice Knight, ♫ Wub Master J ♫, evilgood2010, ♬Puffy♬, Loominarti, ★λDUM JΔN$ON★, surfirecat625, Z46zedboo, misu_misu40, Frank (mr foster), salo_S94, Chompskie, TF2-Reddpie, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, Minimole the Lorax, captsnow, Gun, A Deep Commitment To Konata, Gnome of War, Cpt.Malcon, MarioDox