Peter
The G500 interface to a GTN or GNS uses a combination of ARINC429 (with a Garmin tweak) and a proprietary Garmin RS232. By doing so, it prevents the interface of any non-Garmin GPS units with the G500.
The Aspen EFIS is much more straightforward as it uses industry-standard data bus formats so accepts pretty much any panel- mount GPS and both digital and analogue NAV sources.