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.
re15revisited, [email protected], [email protected], Genma, ..., Seitan, Sora, outgoing4325, Korbenn, ShadowLuigi, Warpath-the-drone swe, jadenwhite50, Daitomodachi, [email protected], LazerSpy, PeculiarSentinel