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.
bednar.michl, JeBuZjAcK, AmGona, Smouky, Yakra, omgthiskid12, Jolas, Nick44, Arnold, Jack O Neill, Loke, Maki, DIFOU, DIFOU, Sabina ♥, Sabina ♥, Matt, Matt, Matt, Morty, Commander Fluffles, Bedinam, LeTexPro, LeTexPro, Blank Shade, Fluffy Eevee