banding seems pretty minimal considering it's a web jpeg, which can certainly be problematic in many areas. Not really a "gradient tool" issue, more about smooth gradients no matter how they were created. this image appears to have a little vignetting darkening the corner, whether it was in post or just the lens not sure.
As mentioned best solution is staying in 16bits (which you can't do when you make a web jpeg, thus the problem).
Sort of a last resort thing to try with banding is adding a little noise to the image ....