Forgive me if I am wrong but I believe this is only binding v6 to the outside edge of VMs. Not GKE. And the document notes you cannot connect to Google API services over v6.
I don't want to overdo the curmudgeon thing: I'm really glad they've started to deploy dual stack, it's long b overdue. And remember that Google has been a strong proponent of v6 in Android and in ietf standards across the board.
Oh yeah, the IPv6 saga at Google cloud is a long running joke. However this is indeed a decent step forward, in particular this is relevant to the comment I replayed to: in order to reach a VM traffic has to traverse the network fabric, which clearly now supports IPv6.
As for GKE that's another pair of shoes, probably more related to configuring calico and friends and less about limitations in the low level network fabric.
As for Google API, I read somewhere that they disabled it because billing exemption wasn't ready.
