I am not a GOS dev, but as far as I know, GrapheneOS based its releases on fixed AOSP tags (like the recent Android 15 QPR2) and also does backports from fixed tags (quote from the recent release 2025032500: backport upstream brightness fluctuation fix from Android 16 Beta 3.1).
If I understand correctly, Google will still open-source those important tags. Just the rolling non-tagged rolling development will happen behind closed doors.
If both of those conclusions are correct, then there shouldn't be much disruption to the development of GrapheneOS.