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.
viento y rayo, Luizgames1.0, kingoros, FIKIP, kauan2005klk, obloxyman, Bobby Brown, elaguasfrescas07, vicentedavidaraya, Aqwertini, SwatmyS, Ydr_8, wonkyhxdo, Sucukpimmel, cappun, deckssiv2, chipiri295reyes, Jvn., jalejandromayorga14, Runic_Flare11, phamvietbinh2k8, sickoof, guzelkuly, milos72062, highnebuladragon, gassole, nottbnrblue, cayeng627, TOB, Tweefey, papucelo, brofistoryPL, JIkali, 私はめちゃくちゃ, Gun_Gale Zero, totaltotal064, luishoal555, razvi, halovetta73, Takashin, Brodo Swaggins, elmamanalansan4, Farca_XD, Farca_XD, jannatijerina7, PN gui wars, tengosidaydiarreamasiva, tengo covid, Z A X, Spitball +9119 others