There's one in the book.
It's also not too hard to devise your own - give each gang a different secret objective, making sure the objectives 'clash' with one another.
(e.g, Outlaws must free gang member from jail, Desperadoes must rob bank, Pinkertons must kill or capture Desperado leader, Lawmen must escort a prisoner to jail (where they will bump into the Outlaws trying tobreak in).
Or you can play in teams (e.g. Pinkertons + Lawmen vs Outlaws + Desperadoes)