Hello So I too got the e-mail regarding the Public Beta GM Candidate. Opened the App Store, went to the Purchases page, and although it is listed the button on the right is greyed out and says Downloaded. Anyone else seeing this issue, and if so have you found a way of making it available. Thanks in advance.
Check in Launchpad if you have Install OS X El Capitan Public Beta downloaded. If so, delete it, and reload the page in the App Store to download the GM candidate.
Version of public beta currently being used is PB 5. Public Beta 6 is still available to download from App Store, but haven't bothered to do so, App Store keeps disconnecting. no El Capitan in Launchpad.
I also received the mail but nothing is showing up in App store. I am on the latest PB (6, I think?).
So App store was recognising a copy of the El Capitan Beta Installer app that was on an external drive. Once the external was disconnected, download of GM could begin, took three days due to the abysmal App Store but now installed and working fine.