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.
esgleal6, [TSSF] Sectapimp, LunarLancer, leonardbob49, riybokon, falseprophet141, fazafreeman, JAN.HYMR, Combine_Advisor1321, Combine_Advisor1321, Jaune3347, iamkaixa, iamkaixa, iamkaixa, iamkaixa, DanielPlays, 왕나비, Blades, AleGamessita [Csgomassive.com], FOR ADOPTION: (Console), DexterSanchezProductions, YeahDoodCzLp, Cremator, Serena, Vengineer, KingBo0?, carlos_zerda, [PDF Sgt.] Mrblue630, sitkinator