The WhatsApp documentation provides instructions for how to switch from iOS to stock Pixel OS: https://faq.whatsapp.com/1295296267926284.
I suggest trying to follow those same instructions with your device running GrapheneOS, and seeing how you get on. You'll need to install the Data Restore Tool from Google which comes pre-installed on stock Pixel OS. https://play.google.com/store/apps/details?id=com.google.android.apps.restore
If you give it a try, I'd be interested to know how you get on. If you get stuck at all, please do ask here, and we'd be happy to provide further suggestions.