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.
[TSSF] Sectapimp, LunarLancer, leonardbob49, falseprophet141, fazafreeman, JAN.HYMR, Combine_Advisor1321, Combine_Advisor1321, Jaune3347, iamkaixa, iamkaixa, iamkaixa, iamkaixa, DanielPlays, 왕나비, morr2234, AleGamessita [Csgomassive.com], [SF] Shadow Ayano Aishi [Muffin], DexterSanchezProductions, YeahDoodCzLp, Cremator, Serena, Vengineer, KingBo0 [G.E.W.P.], carlos_zerda, Mrblue630, sitkinator