Google pay is not working on my device

Please watch my GIF made on iPhone 6 where a transaction of Rs 500 was made using Google Pay.

This says that you are referring to UPI pay and not Google Pay

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

This discussion thread may help you --> UPI Not Supported in iPhone 6s - Apple Community