Well, there's
Sarai-ya Goyou, but it's set in the late Edo period.
Baccano! has a number of gangs, but it's set in Prohibition America. I think the same author's
Durarara has some, too.
Black Lagoon is full of gangs from two-bit hoods through a bunch of battle-hardened ex-Soviet soldiers turned gangsters. The leader of "Hotel Moscow" is a cheroot-smoking
woman in her thirties with horrible facial scars from being exposed to chemical weapons during her tour in Afghanistan. The last arc of the second season throws in the yakuza as well, and it has a most unusual gang leader. I'd give that a
try first.