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.
Unpredictable_God, Captain Jack Sparrow, .#Zipcore, jigor9, Gtayoshi, TomWhatTheGeek, Frost1551, ☆ı ɐKɯ ɹǝDɐן ɐɯSǝ☆, lonely19682005, Ignis, ⌠ℝℴℳᎯ₦⌡, rjledesma, Arctic0ne *nopc*, Destiny | Hyplex.co, rvotri, jdramirezelmo, Gnome of War, Gnome of War, Gnome of War, Gnome of War, *CryingDolphin*