Badnews: The answer is you can't, in your current format. Gearbox is standalone.
Goodnews: You can purchase the gearbox plugin software from www.line6.com
This means you'll have all the great line6 amps and effects and preamps working in garageband.
Snag: It costs $200.
Conclusion: I had the same dilemma. There's no way round it. If you want to use Gearbox as a plugin you gotta pay up (again). Its a shoddy trick by Line6.
I paid the $200. And the guitar plugins are great, but the preamps make it worth it. With each passing day you forget you were stung. But I'm afraid you gotta get Stung.
You can use the UX2 it works very well but with limitations.
Once you have installed the drivers and Gearbox software - you will be able to select it in the GB preferences.
What you must realise though is that the Gearbox software has to be running in order to use the "tones" and effects.
Also you when you select and record you are in effect committing the tone to disk. i.e you can't undo the effects etc!
mmm yep it was a surprise to me too - but the tones and detail in the sound is good - I tend to get the tone and texture I need and add delay/reverb/modulation in GB or Logic Express.
The Plug in Option was released afterwards and yep it costs a staggerring $200 - almost as an after thought it was as though Line 6 didn't get the whole plug in culture.
This plug in will allow you to record a clean signal and then process and undo afterwards but I think it was a poor decision to release it afterwards. My Guitar dealer was sure that the Gearbox software would work like a plug in but it doesn't.
You can use the the UX2 as an I/O box and use GB's guitar modelling - but the Line 6 are very very good.
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Line 6 Gearbox (UX2) into garageband
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