Did the brawn gp car have a Honda gearbox fitted?

No, according to an interview prior to the 2009 F1 season, Brawn said ...

"It is our own gearbox. We converted the gearbox to mount on the Mercedes-Benz engine because all the rear suspension, we wanted to carry that over."

Click on the "Ross Brawn Interview" link below to read a transcript of the interview.