On some displays, it is possible to select a GPS waypoint to waypoint cross-track error signal to a VOR-type display. Perhaps your 'NAV' display illuminates when this the source for the CDI indication?
Oh - and never 'plane'! That's something used by a carpenter. We talk of 'aeroplanes' or generically of 'aircraft'. Those who don't use the Queen's English talk of 'airplanes' though.