It's now available for Public Beta as well. Got a similar message myself.
Same here! Public BETA testers get the GM now, too. Yay!
"Thank you for participating in the Apple Beta Software Program. Your feedback and usage of the
OS X El Capitan public beta has helped us make this release great. We are pleased to give you access to
the OS X El Capitan GM Candidate.
If you are currently testing OS X El Capitan, please back up your Mac and do the following to install the
GM Candidate. Go to your Purchased tab in the Mac App Store and click the Download button next to
OS X El Capitan GM Candidate. When your download finishes, the installer will automatically launch.
Follow the onscreen instructions to complete installation."