rap style generally credited to West Philly hip hop artist Schoolly D, but as for the word itself, his "Gangster Boogie" (1984) used the conventional spelling; NWA was spelling it gangsta by 1988.
Entries linking to gangsta
gangster n.
"member of a criminal gang," 1896, American English, from gang (n.) in its criminal sense + -ster. Related: Gangsterism (1918).