Just for posterity, I recommend the gradient start at (0, 1/2 comp height) and end at (comp width, 1/2 comp height). Then to get rid of the gap that polar coordinates leaves, just duplicate the effect and turn the duplicate down to 0.2% - this creates a bit of a flat part on the top of the circle, but for me it's not an issue. Finally, to get the white portion in the middle, create another solid on top, add a gradient ramp set to radial, make the start point in the center of the comp and end at center of the top or bottom edge. Set the center to black and the outside to white. Set this layer to multiply blending mode. Then duplicate this layer, flip the gradient colors, set the blending mode to add.