Scale Gradient (located at the bottom of "Image Gradient Color Points" section) helps to equalize color transition at each side of the image when following types of gradient are used: radial, rectangle, oval rectangle and diamond.
If image has a shape of a square (width and height have equal values), then Scale has no effect on a gradient transition. If however the width is substantially higher than height for instance, then gradient transition is stretched at the right and left side of the image. To compensate and equalize gradient color transition, check the Scale box. All 4 sides of the image would have the same color transition amount, filling compensated space with center color (gradient point 9).
Round rectangle gradient before Scale applied:
Same Round rectangle gradient after Scale applied:
Note: In case of a diamond gradient type, the Scale has inversed effect - color transition amount would be somewhat distorted.