This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
szabihu2, sterlingeo01, redshooter the box, PuNcAkE™, archery11223344, BangKyu~, Loli hentai cultist, Twitchter, Twitchter, LimezGaming, samanayeur, imchris, VekingGaming | kickback.com, axs | gmod-best.ru, QueB, I'm the cube, Jhonny, -Based- | GalaxyServers.co, Beavers Master, stankydank92, Adrian 更木 剣八 第十一, SenpaiFanta, Baer™, Таверна Любви, xxgamora, Simply A Banana, master4523, mike ames CivilGamers.com, ⠀Slash Element (Alex), YLSgg, squiz v2, [BCRP] Firestorm, Kingsman ###, Singularity Gaming Community, Geopelia, Trump in 2017, WarpZone87, | LordAiked, Barney Sandals, Ilya Volk, TexasTurtle, rodrigovelezdiaz, er nesten kul, °-°, Visual Vincent, ẞԄiԎσ (Brian), CeddySpectator, mhopheeda, Pelly, Anonymous +864 others