Gmail Not Working in iOS 9
Even if you have correctly configured Gmail to work with the iOS 9 Mail app, you may still face problems while logging in.
iOS 9 has issues with Gmail, as reported by several iOS developers.
You may get a message that the username or password may be incorrect… and even if you have successfully logged in, the mails may not load.
Once again, the issue depends on the kind of iOS 9 firmware you are using, and on the type of iOS device – whether iPhone or iPad – you are using.
Solution to this is to use the official Gmail app, wherein you can add multiple accounts – but even that seem to give unexpected errors randomly.
You can use Gmail via Chrome or Safari browser on your iPhone / iPad. Or you can retry or reload Gmail from your Mail app after several hours.