Phone pay goggle pay nhi chal rha

Google Pay does work on iPhone 6 iOS 12.5.3 Please delete the app and re-install. It requires iOS 11 or later.

What does not work is Google Pay UPI.

See the pic below which clearly states you need iPhone 7 for UPI.

Every app has its own boundaries. Please contact the app developer --> How to contact an app developer - Apple Support