Official MIDI Specifications

General MIDI 1

This Specification outlines a minimum MIDI configuration of a “General MIDI System” which defines a certain class of MIDI controlled sound generators. The General MIDI (or GM) System provides a high degree of compatibility between MIDI synthesizers, and adds the ability to play songs (in the form of MIDI data) created for any given MIDI synthesizer module that follows this Specification.

This class of products are intended for broad applications in the music, consumer, and entertainment markets, due to increased compatibility and unprecedented ease-of-use.
RP-003_General_MIDI_System_Level_1_Specification_96-1-4_0.1.pdf
The "General MIDI System Level 1" specification -- also known as "GM", "General MIDI 1" and "GM 1" -- defines specific features of a MIDI sound generator (synthesizer), primarily so that MIDI files are shareable.

Without General MIDI, playback of MIDI files created on one MIDI instrument might sound totally different on a different MIDI instrument, because sound selection in MIDI is done by "Program Number", not a description of the sound. The GM specification assigns specific sound names (such as "Electric Piano" and "Oboe") to each Program Number, but the acoustic characteristics of the each sound are not defined.