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.
goldedplay, kristjan.oru, Dukke Monterier, Shiicho, 1337spy, 1337spy, Alleged Rape Victim, iiAznWolf, dominik_kefel, J-Bear, foxdimi, noobplayer151, Max.exe, NinjaSenpai, NinjaSenpai, The Master [email protected], martin.kosecky5, martin.kosecky5, Dnjido, Tobi76, Red King Christain Van-Fursing, maruhanabachi, Captain Arashiko Tammy, Werwolf_MK9, Werwolf_MK9, Werwolf_MK9, AutisticWeeaboo, Thrax™