Adrian may be thinking of using AirNav's GPS export function but unfortunately it doesn't work unless you buy the GPS (moving map) extension :-( So, we either have to upgrade the license or input the flight plan a second time into the GPS using other means.
As far as I can see this particular GPS doesn't accept flight plans, it just showing your current position on top of a map and it's FlightBook can be used to log flight details, including the GPS route.
The method JayKay outlines is basically what I currently do in FSX, and hopefully soon IRL :-)
Steven.