Gray scale tools are developed to detect primitives when edges is not clear and the image has a gray tones (such as when a front illumination is used).
The tool uses a region to detect the start gray value and then search for the transition in the scan direction.
For example, to identify a circle select Grayscale - Circles, click at the two ends of the area where you want to find the primitive, move the mouse outwardly to surround the area, and then press click again.
Select the newly generated detection in the object tree.
Select the type of transition in the property bar (black to white or white to black) adjust the outer and inner circle so that the tool identifies the desired circumference (you can also change the values in the property bar for fine tuning).
Check if the resulting circle satisfies you otherwise repeat the previous procedure.
Now you can proceed to the measurement as usual.