The information comes via analyst Ming-Chi Kuo, who has somewhat of a mixed record on Apple predictions, although he did predict the release timeline of the white iPhone 4 and the planned discontinuation of Apple's white polycarbonate MacBook.
On the other hand, he was incorrect in July when he incorrectly asserted that Apple would move to 4GB of RAM across its MacBook Air line, and drop the 64GB storage option, moving to 127GB and 256GB instead (although he was correct when he claimed that those drives would remain user-replaceable).
The gold master build of software is generally the final version, although occasionally a rebuilt version must be done. It seems somewhat early for Apple to release the gold master in our opinion, as they have not even issued a Release Candidate yet, and often there are more than one of those releases.
Still, they have 11 days, and they could issue RC1 at any time. Releasing the gold master to assemblers is necessary for the next-generation iPhone to be released. Reportedly, Foxconn and Pegatron have been building next-generation iPhones for Apple, and putting them aside to flash them to the correct software when iOS 5 is final.
Why do we keep referring to a next-generation iPhone instead of an iPhone 5? Because Kuo also revived the iPhone 4S rumor, which has been rumored for months. The rumor says that Apple intends to offer a updated version of the iPhone 4 that has the same general form factor of the iPhone 4, including screen size and building materials, but with an improved camera and tweaked antenna band. It would be similar to the update Apple issued when it went from the iPhone 3G to the 3GS, if the rumor holds true.
iPhone 5 or iPhone 4S, rumors point to an October release, probably mid-October. Typically, iOS 5 would be pushed to older devices a week or two prior to that, if Apple runs true to past history.
Update: There's more validation to the iOS portion of this report. BGR reports that a "trusted source" has told them that Apple will be releasing iOS 5 beta 8 to developers on Friday, Sept. 16. It will follow that up with a release of the gold master "on or around Sept. 23."