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.
catslover565, botheredglobe, Lttaxman, Junɇ, SHADOW, Sorch, Eduardo, fazafreeman, ❤ C R E O N I X ❤, Acecool, Pixieee [LC IS BACK!], Funnygamemaker, Funnygamemaker, [GT] Kampin Karl, ClaytonM456, f.blazevski, Titanium Squirrel, [Oxilium] Luigipower5798, Nidolai, Craftplacer, Erza, Erza, Aim, CPU Renny Heart(EnderNator10), Taylor, Shock, тномаs нunтеr, Sir Mjctechguy, call me father