This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
Gibus, lexmegamozg, gemer 9109, Томогавк, andyguerracid, PickleJuice71, Quality Queen, GKY6887, Dormento, Jenssons