Sean Penn's hamming it up as a Noo Yawk gangster in Los Angeles, Ryan Gosling is being Ryan Gosling (i.e. awesome), and also Nick Nolte is there. What should astound you further is that Gangster Squad is directed by the same gent that directed Zombieland, so the dude's clearly got some range.
This doesn't look like it's going to be a terribly good movie, I have to say, but there's some charm to be had here. We've got what's essentially a black ops FBI team operating without all that fussy red tape featuring a coterie of gentlemen who're reasonably entertaining as actors, and someone's probably going to shoot a tommy gun around and shout "AHHHH," and that's always something to look forward to.