Google pay delegates payment processing to other providers who are charging the merchants fees already. They do not charge their own fees (from https://developers.google.com/pay/api/faq):

Does Google Pay charge any fees?

 

Google Pay doesn't additionally charge users, merchants, and developers additional fees to use the Google Pay API for payments. Merchants, specifically, continue to pay processing fees to their payment processor.

Given that Google's business is about monetising user data for advertising, there are two ways that that they likely benefit:

1. Knowledge of what people are actually buying is very valuable from an advertising perspective.
2. General stickiness within the Google product ecosystem, so they can keep collecting data on people.

Google pay delegates payment processing to other providers who are charging the merchants fees already. They do not charge their own fees (from https://developers.google.com/pay/api/faq):

Does Google Pay charge any fees?

Google Pay doesn't additionally charge users, merchants, and developers additional fees to use the Google Pay API for payments. Merchants, specifically, continue to pay processing fees to their payment processor.

Given that Google's business is about monetising user data for advertising, there are two ways that that they likely benefit:

1. Knowledge of what people are actually buying is very valuable from an advertising perspective.
2. General stickiness within the Google product ecosystem, so they can keep collecting data on people.