One MIDI input carries 16 channels, so that's all you need.

The G2K (and its newer brethren) require something that understands their output--essentially, a Roland interface. Their GR-20/30 etc synthesizers are the main candidates, but Roland also makes a straight G2K-to-MIDI translation box. (Which can you get cheaper? I don't know.) As far as I know, all Roland interfaces let you choose whether to treat the guitar's output as putting notes from all strings on one MIDI channel, or using one MIDI channel per string. (MIDI channels 11-16 is conventional.) I don't know which way would work with BiaB, but surely one will.

There's also a cheap guitar-to-MIDI box around now, the Sonnus G2M. Get that for $100 and a very simple 1x1 MIDI-to-USB interface (I don't think the box has its own USB output), and you're all set. I have not tried it (I use Roland synths). Two reservations: it is monophonic only; no chords, no double-stops, just a single melodic line--which may fit with your purpose. And reviews suggest that there's enough latency so that really fast playing doesn't get translated properly; but to one degree or another that's a factor with every kind of MIDI guitar (except maybe the SynthAxe), and again, it may not be a problem for what you're doing.