From https://developers.google.com/pay/api/web/guides/ux-best-practices looks like we might want to make sure our 'opt-in' checkbox is OK for keeping the donor's email in Civi and mailing them later if we get it from GPay.

Use Google Pay to only gather relevant user data

If you gather data returned by the Google Pay API, it must be used for the purpose of the customer's current transaction only. This includes order confirmation, shipping notification, shipping tracking, order cancellation, refund, and refund notification information.

For example, the Google Pay API returns an email address when you set emailRequired to true in your PaymentDataRequest object.

Maintain this practice to nurture customer trust and loyalty.
Important: If you'd like to use any data returned by the Google Pay API for any purpose other than a transaction, you must obtain separate, express consent from the user, and you must request it from the user outside of the purchase flow.

We'll have to look into how GPay interacts with 3D Secure - My understanding with tokenized cards like Apple Pay was that 3D Secure didn't apply, but it seems to be different with GPay (https://developers.google.com/pay/api/web/guides/test-and-deploy/integration-checklist):

	If you selectively trigger 3D Secure (step-up authentication) for normal card transactions based on risk criteria, be sure to use the same risk criteria to trigger 3D Secure for Google Pay PAN PAN_ONLY transactions.