color wheel in gimp method?

[Ofnuts]

At least two reasons:

1) the first point may not be the one you think:

   

2) The "circle" is not a circle, it is an approximation of a circle. Creating the circle as a path directly (using the various scripts available) makes it a fairly clean (if not perfect) circle, but creating a path from a circular selection makes it even less a circle so there is no guarantee that the local tangent is perpendicular to a line that goes from that point to the (assumed) center if the "circle".