You can root on GrapheneOS. You do it exactly the same way you’d do it for the stock Google ROM:
Have an unlocked bootloader. Yes, this means that it “”“defeats the purpose of GrapheneOS”“”, if the purpose of GrapheneOS isn’t for you to avoid Google’s privacy nightmare. I use GrapheneOS for privacy moreso than security, and not being able to block ads properly is irritating.
Install the Magisk app.
Extract the boot.img from the GrapheneOS image and patch within Magisk.
Flash the patched boot image in the bootloader.
The main annoyance with this is that you’ll have to do that dance every month when a security patch gets released, but for me, it’s better than vomiting from exposure to ads on mobile.