Though Google Pay is also probably the least private of the major tech-company payment platforms (the others being Apple Pay, Samsung Pay, and Garmin Pay). It is, I think, the only one that actually requires an open network connection on the phone to work. The others all generate one-time codes that get sent through the payment machine's network for verification by Apple/Samsung/Garmin on the backend (i.e. you can tap an Apple Watch to pay with all its radios off, but you can't do that with Google Pay).
From what I gather, Garmin Pay can work with GrapheneOS if you have one of their smartwatches. And Privacy.com works, but not with tap-to-pay.
As an aside - I think the Paypal app in Germany offers HCE tap-to-pay, and In the UK/Europe 'Curve' is a Google pay replacement that runs fine on GrapheneOS (and they have first-party support for huawei phones that don't even ship Google play services anymore)