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.
Jacob Rees-Mogg, www.Youtube.com/user/CalvitChin, Fifty Shades of RoyaL, Kurma, >KEKSQUAD, Tude, Bill le Putride, Sorch, ☀ Meme Daddy ☀, MetalPinkiePie, [ATA] Combine Elite, Nimimerkki, Vladmir Orekhov, Vladmir Orekhov, LCPDFR28, xxxNoRtH_KoReAxxx, White Wolf (Portgas D. ACE), тєяяα mdr, lovkal, [UTC]BîøQúâk£, Dr.Fragg, Brick Tamland, [42nd] Chinny