piano scale
G♭ Mixolydian
G♭1A♭2B♭3C♭4D♭5E♭6F♭♭7
Major with a flattened seventh: the scale that fits a dominant seventh chord without altering it.
The same sounds spelled from the other side are F♯ Mixolydian.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
G♭WA♭WB♭HC♭WD♭WE♭HF♭WG♭
What is true of it
G♭ Mixolydian is G♭, A♭, B♭, C♭, D♭, E♭ and F♭, and back to G♭.
Its steps are W – W – H – W – W – H – W, where W is a whole step and H a half.
It costs 7 alterations to write — near the limit of what a key signature can carry.
On the same tonic, G♭ major differs from it by exactly one note.
Chords made only of these notes
D♭mminorD♭m7minor seventhE♭mminorE♭m7minor seventhG♭majorG♭7dominant seventhA♭mminorA♭m7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.