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.
老貓 - TheOldCat, justind383, speznaz97, GotenSSJ, §『㎖i_CRo ζoFt』§ | busy, cypto, zDarKLordZ, ╰☆╮ sarah ╰☆╮, RAINT IT DOWN, ALRIGHT?, DESTROYER OF WORLDS, Baka_Oppai, AnonaMoose, Turner605, [YT]MASLO1337, Michel | Demerit |, l.maire c, Arm'Zii, JigSaw, Chemical, B1GL0LBR, [Woona~<3] Night Strike