GrapheneOS would be so much more interesting if there was an official supported way for rooting it. That's the only reason I haven't switched to it on my several devices (all rooted)
But in that case why you would need to use GrapheneOS at all? Without security you cant have privacy and OS with security as priority cant just add hole in the system because it would allow to bypass all security features added on top of AOSP and AOSP features too. Most features people use root for can be achieved without root by modifying Android Framework itself with SystemUI/Settings app
If you wish so you can gain root privileges on your own in your own build or with modifying GrapheneOS existing builds. It wont be compatible with GrapheneOS provided updates because of signature mismatch
They value privacy and security. Allowing userspace apps to completely circumvent Android's permission system massively weakens both.
If you value freedom to do what you want on your devices, then you may want to consider Librem 5 instead. It runs a desktop Debian derivative with full root access.
That completely goes against what they're working towards. I understand why you would want to root your own phone, but GOS is targeting highest security standards and root ain't one of them (for good reason).