Strange, I've never heard of that. I'm guessng it's a path or permissions issue. Is it safe to assume that all runs well if you execute it from within the main DirectRT program (i.e., using File > Select and run input file)?

If so, it may be a directory issue--something to do maybe with spaces in the folder names and/or DirectRT having trouble following it. First thing I would try is moving your GNAT to a simple path such as c:\gnat and see if that resolves it. If not let me know and we can try something else.