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.
XxAlexXx_Sk9, darks6271, Serafimsoldrei, Tom, zzzzzzzrerere5454, [email protected]$$$TiK..., LIL YING YING CHING CHING, Ludvig, ciguli, Mannotcool, popezjuancito, crazyboyッ, zero, claudioherculano6261, bastiaandejong219, partitigris, artembarhujdaran, ITACHI, S.T.A.L.K.E.R, Koromi Niage, artdedov26, newisboch2003, globalelite175, Jarry, siwakorn7876, iTS ZAGiD, popasos6, reimuuuu, NastkaPL Key-Drop.pl, josecoste192002, hollow knight, WhyDoYouLikeMe, viniciusvilasboas.2007, eduardolugo1221, eduardolugo1221, richardfengtengyu, MOROZ_KAPETS, snezanaantes5, egor8322, ][_Lay_][, dany9hristov9, andreyyyo4 extremium.su, lolnice03, Славефил, Price_Gice, Turhan1030, birdmmon, NoisyDig5876211, deyanpetkov1, gamballyt +2017 others