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.
Dal`tonik, quentinous1, LeTueurBronieDu66, ╰☆╮ sarah ╰☆╮, ╰☆╮ sarah ╰☆╮, Zadix, DESTROYER OF WORLDS, skillz., Andrjuha, [green] Click, Gordon Freemen, Adoring Fan EZ Twitch prime, Mc Cacaraiooo Farmskins.com, Froggers, wheresmygui, [GG]Tense!