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.
Corn_Hub, DARKTR, Midnite, Tanner9078, TheTwoTime, josyalencar34, Mike_Simus, Otsel, Natanskybudder8, DarkShadow5450, YonoAonoY, cannongan294, Danvonwolfen, Tnerb003, [TUD]jerome, yts3088, pimypp, TomBa2005, Mauro., PaL_PaLi4, 蝸牛丨カタツムリ, reyr19, hmz2790, Flying_666, Mr Star, Turtleking14, ddb.diego, Monsier_personne, Tom, Vaporwaves, DasDavid, decode, S c r e e e, TonyPanfilo, skaterlandau, ShadeBeastify58, kocimir.koca, Blazheon, Light, Acid pacifist, scvyy, DearAlone, HonorMeIReformed, jasonmarioland, Marksman EAB, kidbazooka2105, benjaminhenderson254, Lucas_bgl, Eddy, andromoboplay +3208 others