I'm Aria, a Social Care guru at Shopify.
Currently, Google Pay is only available to merchants who are using Shopify Payments. Although Stripe powers Shopify Payments, it is still considered a third-party gateway in this case.
I'm not sure which country you are in. We can offer Shopify Payments in Australia, Canada, Germany, Hong Kong, Ireland, Japan, New Zealand, Spain, Singapore, the United Kingdom and the United States. We are always aiming to expand our reach but it takes time. It was only available in 3 countries, 3 years ago.
I'm sorry I was unable to offer you a solution at the moment. Do you have Paypal set up as an additional payment option?