In reference to the top, curved part of the gate:
To my understanding, the shading of the lower portion is inconsistent throughout the curve. The shading of the parts parallel to the ground is still notably banded, while the curved part's shading has no banding. This inconsistency in shading is what makes the shading look odd and the shape indefinite.
I suggest fully removing the banding on the parallel potions, having the darker shade be mostly 1 pixel high at the bottom to match the curved part. This should make it easier to look at and help define the shape.
(I tried to phrase that in an understandable way. If you need clarification ask.)
To my understanding, the shading of the lower portion is inconsistent throughout the curve. The shading of the parts parallel to the ground is still notably banded, while the curved part's shading has no banding. This inconsistency in shading is what makes the shading look odd and the shape indefinite.
I suggest fully removing the banding on the parallel potions, having the darker shade be mostly 1 pixel high at the bottom to match the curved part. This should make it easier to look at and help define the shape.
(I tried to phrase that in an understandable way. If you need clarification ask.)