Depends.
Current "gangsta" rap? For the most part, I'd consider that a mere trend; a fad. 50 Cent, Ja Rule, even Jadakiss/D-Block etc...
However, I'd say rap "turned gangsta" because streetwise rappers at the end of the 1980's and early 1990's were rapping about what they knew about; the ghetto, guns, drugs, etcetera...a lot of them can lay claim to having grown up in environments that saw various shootouts, gang related ordeals, drug trafficking, etc. etc.