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.
cnpkg, carlosriego1999, ◕Δ◕ StarWolf ◕Δ◕, FORSETI MMIII, FORSETI MMIII, 彼岸花落, LAZER™, LeTueurBronieDu66, DarkShadowDmitri, ╰☆╮ sarah ╰☆╮, [KdH] Terax, [SSoM]DemonLord (Danielson), Jeff Whitey, Jeff Whitey, Artes