Thanks everyone for the comments. Mike, I tried replacing the Gb with G natural, and since G natural is not in the key of Bb minor it sounded out of place. I am not sure what you are saying; when the descending melody hits the Gb, the accompaniment in the background is centered around a Bb chord, so there will be some dissonance as it is a Minor 6th interval ( Bb to Gb) but it resolves down to the 5th. Was it another note that you meant that could replace the Gb?