I am unable to use Google Pay and banking apps on my iPhone 6

Google pay uses UPI. UPI requires iOS 17, your iPhone 6 cannot run anything higher than iOS 12.

Most banking apps require a higher iOS version than what you are capable running.

You will not be able to use those apps on your phone.

Correct. Your iPhone 6, a phone released nearly ten years ago, is too outdated to run most bank apps. For it, iOS 12 is the end of the line.