General MIDI is a standard set of patches designed so that a Standard MIDI file can be played almost anywhere and sound something like what it should. The 128 patches are separated into 16 banks of 8 sounds. In addition, there is a standard drum set.

There's a difference between "General MIDI System level 1 compatible" and "General MIDI System level 1 compliant." Compatible means that a synthesizer provides the full sample/patch set (including the GM drum kit on channel 10), velocity sensitivity, a few basic controller messages, and a reasonable amount of polyphony. For example, Sound Blaster cards are GM compatible when used with a decent patch set (such as the one built into the Allegro game programming library) with nine-note polyphony.

Compliant, on the other hand, requires more than what a mere Ad-Lib chipset can produce in hardware: 24-note polyphony across all channels. Eight-bit and 16-bit Sound Blaster cards need software wavetable emulation (such as that built into QuickTime) to be fully compliant. Criteria necessary and sufficient for GM1 compliance can be found at http://www.midi.org/about-midi/gm/gminfo.htm