As of September 2017, the Garmin G5 electronic flight instrument is not a replacement for the KI-256 flight command indicator. 

The DG/HSI version of the G5 can interface to a variety of autopilots such as the King KFC-150/200 to provide heading and course error to the autopilot.  The G5 can interface to select autopilots for coupled flight in heading and nav modes with a compatible navigation source.

well.  The KI-256 is not simply an attitude gyro/indicator.  It supplies attitude data to KFC autopilots and is a flight director as well.