A new app store policy by Apple has taken effect on June 1 so that all apps submitted to the App Store must support IPv6-only networking (https://developer.apple.com/news/?id=05042016a). How is it possible to host an app on Google Cloud Platform that only supports IPv4 (https://cloud.google.com/compute/docs/networking) and still passes Apple's review process?
The server side of the app doesn't need to be reachable over IPv6 yet. Most IPv6-only networks have a DNS64/NAT64 service to reach IPv4-only servers from IPv6-only clients.
This of course does make your application dependent on ugly transition mechanisms which will (probably) lower the user experience of your customers, so making sure that your servers are reachable over both IPv4 and IPv6 will make things work better. Both Facebook and LinkedIn measurements show that user experience is better from mobile networks over IPv6. The performance improvements range from 10% to 40%, depending on the mobile network.