From the G3X manual:
23.4.9 Lighting
The GDU 45X/46X display and keys can be configured to track 28 VDC or 14 VDC lighting busses using these inputs. These inputs are reference voltage inputs (not power inputs) with a high impedance of ~100 kΩ for pin 43 and ~200 kΩ for pin 26. This backlight level reference signal may be supplied with something as simple as a 10K potentiometer connected to power and ground (drawing 1 mA @ 14 VDC). The center tap on this potentiometer is connected to one of the pins (43 or 26) in the following table. For providing this reference signal to more than one device, a commercially available regulated backlight voltage reference is recommended.