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.
< blank > ?, Yuri, Zuczekxx, erestontoypunto, Neph Da Map Dev, leonkeaven84, leonkeaven84, leonkeaven84, Pandabär, the_mysteroius_sd, Melanore, ShadowDan92, Stanspam, $Ludowyck$, BTTV.ThirdParty, Valmayu, Sporkington_243, Nupero, Naryxian, TheOmegaGun, Stevey Fucking Wonder, paradox, paradox, Joshua66252, h, slava_s901, n6xl36RWTczNx5bZWBsghellcase.com, Saiter, Ms Draca, wabako, Richard, STRoyal.fr | Sharki, Le saloon, Jeff, bruh, Zeo, yung chode, ZC0M, decrypted, Apollon, [SBS] Mr. Tapout, [LC-RP] Andrew, Keotex [MyDarkRP.de], [GL] J. Wilson (Jet), [W-G] ʟᴇᴡɪs, Hop, Putin, Angel, LRBM, Edhilithron Jakob +1322 others