I disagree  The original usage of the term was probably exclusively West Coast but that is no longer the case  Allmusic says Biggie was gangsta rap for example MTVcom calls JayZ a gangsta rapper Nas Busta Rhymes and  Cent are also called gangsta rappers this page calls G funk an offshoot of East Coast gangsta rap this page talks about New York gangsta rap and Chuck D has called both JayZ and Ja Rule gangsta rappers There is perhaps a difference in usage but I dont think the term is exclusive to the West Coast  Interestingly this article is a  google hit which is pretty sweet and a good reason to focus on it I wonder if we could improve it to featured level soon