You have to adjust both the anchor point and the position. For example:
At 50%:
AnchorPoint → 0.5, 0
Position → 0.5, 0, 0, 0
At 75%:
AnchorPoint → 0.75, 0
Position → 0.75, 0, 0, 0
At 100%:
AnchorPoint → 1, 0
Position → 1, 0, 0, 0
Alternatively, you can create a frame that is exactly a width less than the gradient frame, and parent the white pointer to the new frame with an anchor point of 0, 0.
The green frame is the adjusted box, black is the pointer. Notice how the adjusted box is smaller than the gradient box by exactly the width of the pointer:
This way you don’t need to adjust the anchor point, although the pointer may be inaccurate in some cases.