I was curious about this question, so had a go and installed the Data Restore Tool onto my device to test it out. From first appearances, I don't think the Data Restore Tool is an app that you can open. It is more that it works in the background to support other apps, in this case WhatsApp.
Have you tried following the instructions step by step instructions exactly as provided by WhatsApp. I can't see that WhatsApp asks you to open the Data Restore Tool app, it only states that it needs to be installed.
To be honest, I am not 100% sure that it will work on GrapheneOS, but I am interested to hear how you get on! You may need to give both WhatsApp and Google Play Services various permissions including contacts and media. WhatsApp probably can just use the storage scopes and contact scopes features, but I believe Google Play Services will need the full contacts permission.
If you are still stuck after following the instructions from WhatsApp, you can always ask again for help. Another option, would be to transfer your WhatsApp data to a device running stock Android if you have one available, and then from that device to your Pixel 8a.