I too don't mind the dark underside in this case...and leaving it as is helps keep the bill nicely outlined against the wing. I like the gradiant sky. I'd prefer the gull a bit higher in the frame though.
Good eye contact and detail. I agree that trying to brighten the underwing would probably just accentuate the noise, and so the image is about as good as it can get, but the contrast especially on the bird's right underwing is a little more than I'd prefer. The plus as mentioned already is that the bill is accentuated in front of that dark area.