I got gearbox to work on sierra =)
1. Install the new audiodrivers..
2. Use a program called Pacifist (http://www.charlessoft.com/)
its a shareware program and opened the dmg file to show all the contents
3. Open the Gearbox.mpkg in Pacifist to install the packages one by one... BUT!!! Without the files already installed and without installing the old audio drivers ofcause =)
Try that..
Cheers! :P