From the Oxford English Dictionary:

Contraction of med.L. gamma ut; f. GAMMA the name of the symbol (introduced in the Middle Ages to represent a note one tone lower than the which began the scale inherited from classical times) + ut, the first of a series of six syllables used as the names of the six notes forming a hexachord.

1. The first or lowest note in the mediƦval scale of music, answering to the modern G on the lowest line of the bass stave. Obs. exc. Hist.

2. The ‘Great Scale’, comprising the seven hexachords or partial scales, and consisting of all the recognized notes used in mediƦval music. It extended from ut (= G on the lowest line of the bass stave) to E-la (= E in the highest space of the treble). Obs. exc. Hist.

3. Hence in later use: The whole series of notes that are recognized by musicians. Sometimes also used for: The major diatonic scale, or the ‘scale’ recognized by any particular people, or at any period.
b. The compass or full range of notes which a voice or instrument is capable of producing.

4. transf. and fig. The whole scale, range, or compass of a thing.