Google has apparently dropped push notification support for Gmail on Mail app on newer Apple iOS devices.
Users who have it working right now are grandfathered and would continue to get the push updates.
They would lose access if and when they switch to a new device.
The support was interestingly dropped back in January. Google of course has an official mobile app for Gmail out on the Apple App Store which offer this functionality. Looks like they want users to switch to Gmail app.