What mode starts on G and has one flat, Bb?
The mode that starts on G and has one flat, Bb, is the G Dorian mode. The G Dorian mode is derived from the F major scale and consists of the notes:
G, A, Bb, C, D, E, F, G
This mode is the second mode of the major scale, characterized by a minor third and a major sixth relative to the G root note, giving it a distinct, slightly dark yet hopeful sound.