If you want to record audio directly from the USB audio device of the GR-55, it must be selected as the current audio interface in Reason Preferences.
Reason does not allow multiple audio drivers at the same time because the science of digital audio clocking means most combinations of audio interfaces malfunction if not properly clock synced.
If you want to be sure you are recording a intact audio stream, I suggest routing the analog monitoring signal out from the GR-55 into your listening equipment while recording from the GR-55.
You can try how well your combination of Mac built in output and GR-55 input will work, by setting up a Aggregate Device using Mac OS X.
The GR-55 probably uses an internal clock, since it can operate without a computer.
Theoretically, if the Mac built-in audio interface is clocked by its Mac OS X driver and not a crystal oscillator, it could be driven by clock recovered from the GR-55 and the two interfaces would work in sync. However this would require the Aggregate Device driver to recover the clock from the GR-55 and use that clock to sync the driver for the built-in audio interface. I doubt that Mac OS X Aggregate Devices can pull that off.
Edit for completeness: In any case, the Mac OS X Aggregate Device driver can artificially sync any two interfaces, but unless they can truly share a single clock, the result is destructively modified audio. In some cases, such destruction is not audible, but is always unavoidable.
Last edited by jengstrom; 2012-12-06 at 12:20.