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.
Jas, Corzza, Cameron, Tvoi timmeyt, kuba-popko, araihanassyam, Джони Сътъра, ilm9001 (QUACK), ricohallo, My Name's Not Alfie, [CORRUPTED], xnikeman910x, siardypma, TDR-Akame_Vi_Britannia77[OSU!], Mob1234, Mr.Apple, bardexcz, SuperButhTubMan, Nobody, LeTueurBronieDu66, ╰☆╮ sarah ╰☆╮, tammba74, rm7gaming, skillz., Neptune (#TeamPyro), TheGamingCharizard, Cryxia, King Leoric, Vitality, WinstonC | trade.tf, MyNamesJeff, Mo15, Marc Carless, GMcrafter64, Abood | kickback.com, kyle.stokes321, |RG| GoldenPuncake, Dare, Thomasthomas2003 #TeamPyro, lucasvdorr, 감귤 작작먹어, [NOR] Mitch Musicman