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.
mrbeast, emanuelortiz588, Sansyxxxxx, karim, gar.yenos420, Lil_Eze_08, Suey で-, virtualfrog1000, Artur__Vargas, Artur__Vargas, matthewjsiron, Clippy_xp, renditile2, Saman.exe, iginhoplace12, supakornkuttiya, crisamavini, ANTON O.W.N KING, Vessot, lionelver1221, C H I M P, atadan2662, yes father, jst123KeyDrop.com, norah.c298, Leon kennedy, erproduction.639, @zaz3l, Donny Jimbo, iambox20776, Strelok, medenüz :D, Pine_Apple_Jews, luisdyer15, alanskywar, kevjames523, LeSaucyBoi, 0825257118a, joshuapapaeliou, uauanisabucken, SuperPlayerHK, Staid, Waluigi_WAH, obstaculizar10, obstaculizar10, izibot.krushitelevich, guna9579, chekakan, weebwarrior22, ngijnanade +375 others