Google Pay will work, but NFC payments through Google Pay will not work, as that specific functionality requires a Google certified OS.
There are other apps that do contactless payments that don't require a Google certified OS, and those will work fine on GrapheneOS.
GrapheneOS will almost certainly never be certified by Google because it doesn't pass all CTS tests because it does non-standard things like adding the Network and Sensors permissions.
Any app that wants to use SafetyNet / Play Integrity API but whitelist GrapheneOS can do so with the guide that GrapheneOS provides, so it is up to them to support the OS:
There are technically ways to bypass weak checks that don't do hardware attestation, but that will stop working as soon as things move to hardware attestation, and there won't be any way to spoof things to get those apps to work. It would be worse to get those apps working temporarily and have users expect them to work and rely on them only for them to be permanently pulled away from them once things move to stricter hardware attestation.