I wouldn't class Brother Ali as gangsta rap at all really Baley, in fact some of the music I have heard from him has been quite good, as far as rap goes.
Surr, Gangsta rap in my and a few other people I know eyes is rap made by people kind of like Eminem, which is just rap about killing, stealing and shooting and general things you would see on GTA: SA. Generally it doesn't include much clever poetry, just the rappers grunting and complaining about how their stash has run dry to beats. That's the way i see it anyway.