So what is the difference?
GI275 contains everything inside the unit (no extra boxes are required for the standard installation ADI/HSI). Its mounted from the behind in an existing instrument hole, it’s the “real thing” when it comes to build quality solid aluminium chassis (in some cases heavier than a traditional VAC driven directional gyro), TSO certified, it has Connext wireless capability, touch screen, internal VFR GPS (if enabled), capable of displaying synthetic vision (option), internal navigation/terrain/obstacle databases, can interface to almost everything and if you have 2 ea GI275 ADAHRS you can remove existing airspeed, altimeter, turn coordinator, vertical speed indicator and possibly also existing NAV CDI’s.
The Garmin G5 need’s a separate OAT module (GAD13), it need’s a separate ARINC adapter (GAD29B) that connects the GTN/GNS GPS and NAV to the G5, you will need to keep the basic T intact (airspeed, altimeter, turn coordinator and vertical speed indicator can not be removed). However the G5 has 3 advantages over the GI275 and that is excellent integration with the GFC500 autopilot (the G5 and GFC500 is practically built as a system made for eachother), much lower weight (the weight of the GI275 may be an issue in some aircraft instrument panels) and the lower price? Well the G5 is quite expensive compared to the GI275 if you also take into account all the features that the GI275 EFIS offers.
One common good thing for both GI275 and G5 is that you can most likely remove the existing VAC system. (G5 can not emulate the attitude signals that the Century autopilot requires but then again, a Garmin GFC500 autopilot is the way to go).