The quick answer to your question is that both Real Band and BIAB can generate MIDI parts just like they generate REAL TRACKS.
In BIAB some of the styles are RT styles, and some are MIDI styles. The MIDI styles generate MIDI tracks, and a MIDI track can be played thru any soft synth (Which is what GPO is)
That's step 1, knowing what is possible
Step 2 is knowing how to do it.
here's a link to the tutorial:
http://www.pgmusic.com/tutorial_garritan.htm