As an afterthought - there is nothing to "see" in ST GUI UNTIL after the first GM MIDI file starts to play through it, then ST will populate the part "slots" (1 upto 16 parts) on the left hand side of the GUI with instruments.

As a FINAL afterthought if you open BIAB, set the "Default Synth" slot in the "Plugins" pulldown to ST 2.5 (the VSTi; stay away from using ST DXi it causes some issues), set up ST with OmniSynth 2 with the GM "script" (STMP) file as described above (once you figure it all out) and as long as you don’t change the Default Synth then ST 2.5 will always load with BIAB and be ready to go as a GM "module."

Larry