The movie is based in a 1949 Los Angeles. Gangster Squad's main plot is centered around a gangster named Mickey Cohen (Sean Penn), a criminal who owns the criminal underworld of Los Angeles. The police cannot stop him because no one will testify against him and he's got a good deal of the police force on his payroll. In order to stop Cohen, Police Chief Bill Parker (Nick Nolte) orders a heavy-handed Sergeant John O'Mara (Josh Brolin) to put together a vigilante squad that will help take down Cohen's criminal empire. The other members of the Gangster Squad include: Sergeant Jerry Wooters (Ryan Gosling), Navidad Ramirez (Michael Pena), Conway Keeler (Giovanni Ribisi), Coleman Harris (Anthony Mackie), and Max Kennard (Robert Patrick). Each member offers a different useful aspect to the squad. The squad mainly used guerilla warfare of sorts in order to carry out their missions, gradually chipping away at Mickey Cohen's empire.
The movie was good, but nothing really more than an action movie. It offered in your face type of action, but the plot itself was predictable and not exactly creative. However, that just is usually the case for action movies. The actors did a great job, and the movie was good for face value. There's nothing wrong with the movie, it just wasn't exactly a gem. Describing this movie, I would use the phrase "pretty good" meaning it's good, but not really anything special.