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.
poipon, Saysay, RAITOKI, 餅太郎, Shadowfled, kubosan125, Hinanai_Tyobi104, starsla4, [JP]Nekozeryu, Raffica, treasure-mile, となりのスライム１１６, izak, nekotunnel, kome342, garnet | Coockie1173 | VHS7, chvassumpcao, UMA1231, nuclearirbm, Jacket, 1Dragon234, yutanlikesgame137, dada_yacine, zaki8219.com, FNG_xMATTx