Yup... good advice, you need to select the appropriate input in both the audio / midi setup, and in GB. Then, you need to turn on monitoring in GB.
If you can't hear it, in the dialog box where you turn on the monitoring, set the input to channel two (mono).
A couple of other words of advice:
1) The built in audio input on Macs is unpowered, so unless your guitar has active electronics you will need to preamp the signal...unless you are happy with mediocre tone. Apple is a bit misleading when they say guitar straight into the Mac. You can do it, but the tone bites.
2) If you are buying a preamp device such as a pod, or a M-Audio Fast Track, all is good. If you are taking a line out from your amp, however, make sure it is from the line out port of your preamp. Do NOT take a speaker out signal and plug it into your powerbook.... you will fry things!
Cheers!