Ofcourse, The Godfather (1-3).
Once Upon A Time in America, too.
Yes, Scarface, The Untouchables.
Also, Blood In, Blood Out.
The Outsiders, but it was only a gang movie.
Can we consider Bonnie and Clyde gangsters?
Try: A Better Tomorrow (1 & 2) and HARDBOILED (Directed by John Woo, starred Chow Yun Fat).