It's blacking out due to the GPS failing, the gear position sensor is the circuit that controls the illumination. Should the GPS stick between gears or not make contact every time, the display will go out.
The display turns on with the ignition switch because the GPS is getting power with the switch. The display will go out if the GPS is not registering a gear.
Those who have one should review your installation instructions, you will see that the power lead must be connected to the battery or a constant on. I'm not ****tin' you guys ... Putting the power lead on a switched circuit will not work.
I try hard not to sound like an A-Hole on the GiPro topic, but you guys scare me sometimes making assumptions about it.