Agreed that BIAB should include a revamped, and 64 bit, GM synth. One big reason is that biab users like to share song files, and there needs to be a standard against which they are heard. pgmusic needs to have demos which show "this is how it sounds".
There are two big differences in how to proceed.

Quick and dirty: There are many public domain soundfonts, but I'm sure pgmusic could license one cheaply, if not created on their own. A 64 bit compatible sampling engine with a UI would be fairly easy to develop. This is any easy task with the tools out there these days.

An elegant solution. The real opportunity to shine would be having a GM synth which has some amount of performance intelligence so that the CC's people talk about optimizing are set by either BIAB or the GM synth(++) itself. It could even get its cues from the BIAB styles. Developers could devote a tremendous amount of time this type of approach, but once the initial hurdles were crossed, a big chunk of the results could be realized in a manageable chunk of development time.

Finally, I have found a cool signature with sufficiently dry humor.