The following are links to companies who make automatic GPS based flight control systems for model aircraft.
(Best appears to be the MicroPilot MP2000. It comes with all necesary sensors and controls for flight stabilization, GPS guidance, automatic logging, Manually piloted, Manually directed and full autonomous flight modes, telemetry output (requires your data radio transmitter), waypoints of route can be changed in flight, automatic switch to autonomous mode on loss of RC signal, Can return home automatically on low battery , or on low fuel.
UNAV offers similar equipment. This approach is more "build it up bit by bit".
If anyone buys this equipment, please report to us on your experiences with it.