From the GM release, I ran the final release installer. It took it's sweet time just like upgrading from a more distant OS, but came up OK and now I'm on Version 10.11 (14A284), a slight increase in number over the GM release.
I did have to fix audio after the update.
I'm still being prompted by software update to install the 10.11.1 beta. Their doc page said if you installed the final release you should be out of the beta program, but it apparently ain't so. I guess I have to get out another way.