Developer Portal
6 Managing Google Pay Data Collection
The included Google Pay JavaScript provides a section that controls the collection of payment data by your application. This controls the payment sheet and required fields that are presented when a user processes a payment with Google Pay. This can be set up to collect a minimum of cardholder name and zip, or full data including phone number. Note that requiring all data may require the user to modify their Google payment account since the Google Pay page will enforce your requirements.
All request and reply fields are documented by Google here: https://developers.google.com/pay/api/web/reference/object#PaymentOptions.