This download looks like a map. You need to place it in your maps 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.
caillesjamesleon, [L²:RP] Flashplayer, [email protected], benjaminl1984, Hey ! Mikey.S, pippengerrobertdalton, Lensch, JoshDBoss, Riha.Jarous, Taradosaurio Rex, kid_sphinx, OMGITSMOM, Stoned Apples, sirluigo, D4BTactics, gold94chica , willthyawesome, DustyDan95, DustyDan95