How to adjust gamma correction on old monitors
With your monitor connected, go to System Preferences &gt; Displays and click on Calibrate with the Option key pressed down. This turns on the otherwise absent Expert Mode in the Display Calibrator Assistant.



This will present you with far greater control, including on about page 7 a Gamma adjust.



Nothing can replace a proper hardware colorimeter for screen calibration, but this will at least allow you to get closer.