GB only recognizes metadata on files formatted as Apple Loops. If you want GB to recognize your metadata, you will first need to convert all your files to Apple Loops.
You can do this by dragging/dropping each file into GB onto its own track, and then (with the track selected) go to Edit --> Add To Loop Library. Fill out the appropriate info in the menu that pops up, then click Create. Your file will be converted into an Apple Loop (aif) and should appear in Finder under Library/Audio/Apple Loops/User Loops (at least that's the right folder for GB '09).
Good luck.