You might want to try the band library at Central Missouri State University in Warrensburg, MO. I was the band librarian there several years ago and I recall seeing the band parts to the Gregson in there.
The website for their music dept. is www.cmsu.edu/music. The best person to contact would probably Dr. Robert Gifford, who is the Director of Bands there this year, and the former Director of Bands at Southeast Missouri State University.
When I was there, we would loan out pieces occasionally.
Hope this helps!!
Jeremy Knudtson