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.
Topsuey, alejandro_larcher, kujoperek82, БОЙКО, sahanmehmetcan56, osuruk-kokulubok, re15revisited, szathmariadam28, cjgrama, giga333, Col. Mustard, jwnovosel, Nick Hertz, stG |™.: RedField, stG |™.: RedField