Ha, I just got one of those, nice aren't they !

Check this out:
http://www.guitartricks.com/lesson.php?input=8471

In a nut shell you are best off running an S/P DIF out of your GT6 through a coaxial to optical converter (like an M-audio CO2) and into the TOSLINK ports at the back.

Failing that go analogue, get a lead that will condense two mono 1/4 " jacks into a 1/8" TRS stereo jack which will go into the line input in the back.

Then all you need to do is configure the input in the audio section of the system preferences.

Core audio man, it rocks !