As you know, the gradient is a vector. So you can get magnitude and direction from imgradient(), or get the direction as x and y components using the imgradientxy() function. The help for imgradient() says
Gradient direction, returned as a numeric matrix the same size as image I. Gdir contains angles in degrees within the range [-180 180] measured counterclockwise from the positive x-axis.
So the vector could be pointing in a negative direction, for example if you had an edge where it's dark for upper pixels (top part of the step in intensity) and bright for lower pixels. For example if the image had the top half be all 120 and the bottom half pixels all had the value 150. The gradient right at the edge would be 30 and it would be pointing downward (-90 degrees).
Does that explain it?
