The update, build number 15A282b, is available through the software update mechanism in the Mac App Store and through the Apple Developer Center.
It is not clear if the OS X 10.11 GM will add any new outward-facing changes to El Capitan, as many of the prior betas have focused on under-the-hood performance improvements and bug fixes to optimize the operating system ahead of its public release.
During the beta testing period, Apple seeded eight betas of OS X El Capitan to both developers and public beta testers. The final beta, beta 8, was seeded two weeks ago, on August 31.
As an update complementary to OS X Yosemite, OS X El Capitan builds on the features introduced last year. Behind-the-scenes improvements in El Capitan make a number of apps and processes on the Mac faster, and the introduction of Metal makes system-level graphics rendering 40 percent more efficient.
El Capitan introduces a new system-wide font, a revamped Mission Control feature, a split-view feature for using two full-screen apps at once, deeper functionality for Spotlight, and improvements to Safari that include Pinned Sites and a universal mute button.
OS X El Capitan will be released to the public on September 30.
Update: Apple's pulled the OS X 10.11 El Capitan GM, and it's no longer available for download. In the Developer Center, it's listed as "Coming Soon."
Update 2: The El Capitan GM is once again available for download. Apple has also begun sending emails to public beta testers letting them know the GM is now available for beta testers in addition to developers.