The G5 backlight is either controlled off of the photocell or manually. The G3X can be controlled manually, through the photocell, or using a dimming bus.
If you have a dimming bus for the G3X and have the G5 on photocell, you could dim the G3X to match the G5 on photocell.
If all products are on the photocell, you would need to sit in your cockpit and adjust the lighting bus values on both the G3X and G5 through various lighting conditions. The values are immediately stored and the dimming works in configuration mode. However, I do understand that the effect of different colors on the display in normal mode may make it harder to balance brightness when looking at the configuration mode screens. I would try to balance them as closely as you can through the brightness range in configuration mode, then check it in normal mode.
Thanks,
Levi
Team X Project Manager