I just got GearBox Version 3.72.0 working on El Capitan with my Tone Port UX2 with the following steps.
1. I did a clean install of El Capitan formatting the HDD and migrating user settings but NOT the applications from a Time Machine backup (of a Mavericks setup).
2. I installed the latest Tone Port drivers (Line 6 Audio-Midi Driver 7.5.7.dmg) and confirmed it worked with Garage Band.
3. Instead of running the GearBox setup which doesn't work, I recovered /Applications/Line6/GearBox.App from the Time Machine backup of Mavericks.
That's it. My old saved settings were intact too. I also installed Line6 Monkey but I don't think it was necessary.