To start, I believe the song "Jazz Thing" was conceived by Branford Marsalis and/or Terry Blanchard. Point is, it wasn't that Gangstarr decided to bring in live horns on one of their tracks...it was that a jazz musician(s) decided to collaborate with a rap group. When labeling this song as the starting point for a genre, this is a great distinction.
But even then, there was a live jazz/rap song that preceded "Jazz Thing" by half a decade. In 1985, Cargo issued a song fittingly (and not so creatively) titled "Jazz Rap".
By the time Jazzmatazz and Buckshot LeFonque came around, there had been other live jazz/rap records; some by jazz musicians, some by rappers. Gangstarr as a collective and Guru and Primo individually certainly played a key role in advancing the genre, but my point is only to say that they did not "invent" it.