According to a memo released by Nuance and other "sources" the GM will probably be released on September 10th which is the same day of the Apple press event for the new iPhones (and possibly other products?) As for the public release of iOS 7, current rumors suggest that it will be around the time or just before the new iPhones ship which might be on the 20th with the release of iOS 7 coming out a few days before on the 18th. But we probably won't know for sure until Apple announces it on the 10th.
If I had to guess, I'd say GM on the 10th and public release on September 18th. Apple traditionally launch iOS x.0.0 versions a couple of days before the new iPhone launch.
It may be later for iPads, October time with the iPad 5 and mini 2 launches.