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.
petukhovdmithry2018, terrycrewsnipples, snake.kain, Hardirent, 4am.joshsummer, nightrainmusic, ☺madBunny☺, [GaZe] Shritler, #NOFORM Kubowc, UltraDonovanIP, E7ajamy (home, free to play), Vybre, FoxHound1998, Edmond, Frost177, Trygveblacktiger, ßácõñ, LaserOfTheDie[TR], Baron Sludge of Nova Prospekt