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.
☯ Zeyther ☯, Sloth, DTHD I daanh, (Myg0t)crazyzultan, B4K3R_245, Akisath, [SAPER] MrBartosXD, zDarKLordZ, Saints_Hunter, ZombiekillerRH, TF, TF, Bolzer, ThaBlackillusion, Victorian Seagull, [MLG]politieguy, [FR]Digi, MrBadassPiggyface, mTw, Ark, ogie