The OS has a built in synth. If you double click on any of the MIDI objects (noteout, pgmout, etc.) in a locked patcher they will put up a menu of what is available to be controlled. On the Mac the built in synth is called "AU DLS Synth"
I think it’s 1-128 actually, rather than 0-127. Instruments 0 and 1 should be the same thing (Acoustic Grand Piano). You can readily Wiki the full GM instrument list. Also note that channel 10 is drum sounds, so instead of each pitch being the same instrument, they’re each a distinct sound. And here, the only available pitches are from 27-97, if I remember right.