More information, please: does your MBP have a headphones jack and a line-in? Then use the line-in and select it as the input in GB's preferences. Does it only have one jack? Then you can switch between using it for headphones or as a line-in:
http://howto.cnet.com/8301-11310_39-57459083-285/record-through-your-macbooks-he adphone-jack/
Having said that, your microphone's output probably doesn't have a sufficient level for recording through a line-in port - you'll have to use some kind of pre-amplification, e.g. with a mixing console.