We love FlightGear and all that it stands for, but the truth is that it's a bit complicated for beginners. For the rest of us, there's the old stand-by, Microsoft Flight simulator. There's a free plug-in for MS Flight Simulator 2004 called GPSOut that simply outputs GPS of the current flight to the serial port of your choosing, with whatever NMEA sentences you want. If you've got FS 2004, download that, copy the dll and ini files to your Flight Simulator modules folder and edit the following lines in the GSPout.ini file so that they read like this:
Sentences=RMC,GGA [add any others you want]
Port=COM2 [change to whatever port you're using for your serial connection that substitutes for your GPS]
There's no sign of the plug-in in the FS program when it's running, but if you've copied it to the right directory, it should be silently outputting the GPS in the background while you're flying. So fly to the area where you'll be testing your real UAV and you should see the correct GPS data streaming into your Basic Stamp autopilot.
Here's a video of this working (that's me and my ten-year-old):