If you have wiring diagrams for both, then yes, it's possible. You'd have to add the circuits and connectors for the GPCM to the 03 harness. But it will be a lot of trouble and the potential for screwing it up is pretty high.
Good luck finding a new harness for a 7.3L. They're not produced anymore by Ford (or probably anyone else for that matter).
What's your specific issue? If you haven't had an engine fire that destroyed the whole harness, you should be able to fix the issues with new wires and splices on the affected circuits. If it's connectors, lots of those are available as new pigtails from Ford (Link), or there's the wrecking yard approach.
I wouldn't shy away from adapting the '03 harness, but I've done quite a few things like that, and do have the resources and time.