Etymology of gamut

The word gamut originally "lowest note in the medieval musical scale," in the system of notation devised by Guido d'Arezzo, contraction of Medieval Latin gamma ut, from gamma, the Greek letter, indicating a note below A, + ut (later do), the low note on the six-note musical scale that took names from corresponding syllables in a Latin hymn for St. John the Baptist's Day:
Ut queant laxis resonare fibris
Mira gestorum famuli tuorum
Solve polluti labii reatum,
etc. Gamut came to be used for "the whole musical scale;" the figurative sense of "entire scale or range" of anything is first recorded 1620s.

In modern Greek:
a) gamma: the Greek letter gamma [Gr: γάμμα; γ ]
b) gamma: gamut, spectrum, range [Gr: γκάμμα]

 OED.
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