The issue is apps banning using a device not licensing Google Mobile Services or a non-stock OS via the Play Integrity API. Google Pay does this and a lot of banks outsource tap-to-pay to Google Pay instead of providing their own like many European banks. GrapheneOS users in Europe have multiple options. Users in the US often use a smartwatch for this purpose which includes the option of Garmin Pay rather than only Apple Pay and Google Pay.
The choices depend on the region. It would be nice if the Play Integrity API was forced to permit GrapheneOS via hardware attestation verification by regulators. We're pushing for it in Europe.