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.
Random Sans, >KEKSQUAD, Tude, Waykko, Sorch, MetalPinkiePie, Nimimerkki, LCPDFR28, xxxNoRtH_KoReAxxx, White Wolf (Portgas D. ACE), TerraPlay#QLF, lovkal | trade.tf, [UTC]BîøQúâk£, Dr.Fragg, Itta PuPu, [42nd] Chinny