Nice post, crabblion
As Awesome writes, it's more about corrupt cops, but it's got quite a few gangsters in it and a kind of gangster movie feel, so I think it should qualify!
Since we've mentioned the 90s and a gangster film with Kevin Spacey in it, I should of course also mention the brilliant "The Usual Suspects". Again, I'm guessing you have seen it because most people have, but if it turns out you haven't, well .. You should and I'm sure others can also vouch for this one!
It's a bit hard to summarize the plot, so I won't, but if you haven't seen it and think you might like to, I advise you not to read up on the movie first. It's very good when you don't know what's going to happen.