Same monitor - same problem.
I have a Lilliput 8" Wide Model 889GL-80NP/C/T, using RR and it's built in gamma control.
At night, the brightness of the screen is still too high.
It is not secure. People from other cars or pedestrians can notice the screen very easily at night.
So, what settings are you using in your gamma configuration? Or what else are you configuring or doing to reduce screen brightness at night?
PS: At day normal or high brightness is important.
Setting gamma does not reduce the backlight brightness, just the transparency of the TFT panel.
For reducing the backlight, wire a condenser (2.2kV, 22-33pF) serial into the backlight high voltage wire. At day, you have to short condenser pins for normal brightness with a switch.
Be careful about the high voltage shock risk, insulate the components as well as possible!
If you are handy, apply some white leds around the backlight unit from sideway and switch the normal backlight off. It needs a voltage approx 0V at a certain pin of the backlight unit - switch this point without any risk, because it is low voltage.