If your output gamma is correct (linear, also called "1.0"), you
will see four boxes. If it's incorrect, you'll see 8 or 12.

If the gamma correction values are too low, the boxes on the inside
will be brighter than the surrounding boxes.

Start by adjusting the "normal" gamma correction, using xgamma -gamma [value]
Default is 1.0. Try 1.6 to start.

On the White-Gray-Black box make sure you can differentiate all
grays and that white is as light and white as possible. And that black
is black and not some dark gray.