Understanding Gamma 2.2
Gamma describes how a display transitions from black to white. A Gamma of 2.2 is the standard for the web and professional photography. If your Gamma is too low (e.g., 1.8), images look 'washed out' and gray. If it's too high (e.g., 2.6), you lose detail in the shadows, a phenomenon known as 'crushing blacks'.
Gamma vs. Brightness
Brightness affects the black level (the darkest part of the image), while Gamma affects the mid-tones. If you change your monitor's brightness significantly, you should re-run this Gamma test to ensure your color curve is still accurate.