Contactless payments already work on GrapheneOS via Curve Pay, PayPal and the apps of many European banks. Solving the duopoly between Apple and Google for smartphone tap-to-pay in the US isn't something GrapheneOS can do.

Regulators / legislators can force Google to let GrapheneOS pass the Play Integrity API checks and Google Pay will start working.

Consider contributing to the continued existence of cash by using it.

Curve pay works great btw!