The monitor should be turned on for several minutes before making this adjustment. Setting it lower loses image detail. The top squares should be invisible or nearly so, the middle should be slightly visible, and the bottom should be distinctly visible. You must uncheck it. Value is normalized to a maximum of 1.
Contrast is the monitor contrast setting.
I wish I'd thought of it first. I'm reproducing it here with his kind permission. This chart enables human eyes to determine a monitor's gamma at each of 9 distinct intensities. The gamma matches where there is vanishing intensity difference between the solid gray rectangles and the stippled horizontal strips. But with this chart, you can judge for yourself.