A draft Applescript which I have used to copy my data from iGTD to Omnifocus. No guarantees that it will work on your system, but it should at least be a useful reference.

The code is intended to:

1. 1.Place iGTD inbox items directly in the Omnifocus inbox.
   

2. 2.Place iGTD projects and their sub-projects and actions in a date-stamped Omnifocus import folder.
   

3. 3.Place iGTD archive categories and actions in a date-stamped Omnifocus import folder.
   

Note that it runs slowly - you may need to go away and let it cook for a while.