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.
✪Zero-Trades✪, TREESHAUN, bonniesemface1.5, abcdef.1134, omaigot22, zoldosdaniel, Nimblex, DAVID4GAMER, MingeBag, bflmn, fpsgerald, StrickenNZ, SonicBraine, Zombie Gangster, ollarpatrik2001, robocock, Dragicula, Lemony Link, bunnyskin, gaga2519, GravityTrash, Willymont, Mizcoinferno, killer keemstar, Cä§për®~FYANB~, Valkyrie