The GT6 does amp modeling, right?

Without investing in a great deal of extra gear and mics, you're probably best off going direct from the GT6 to one of the G5's audio-ins (for the short term, the analog in is probably your best bet).

From Apple's website:

Optical Digital Audio
The Power Mac G5 comes with audio capabilities that are not usually found on personal computers. The state-of-the-art optical digital audio in and out ports use the S/PDIF (Sony/Philips Digital Interface) protocol over Toslink cables for connecting to devices such as decks, receivers, digital instruments and even 5.1 surround-sound speaker systems. And since S/PDIF is an optical signal — a beam of light — you won’t get any ground loop interference caused by copper wiring.

Analog Audio
The Power Mac G5 also features high-quality, analog stereo audio line in and line out ports.

EDIT: You're probably best off listening to Dr. Simon. Damn him for hitting "submit" faster than I did.