Just finished (ah - I'll never be finished ;) ) setting up my new Pixel 6 pro on GrapheneOS.
Part of this setup involved setting up Google FI (first time) and porting my number to FI from Verizon.
What I learned is - to setup Google FI - I needed to do so from the stock android. I could not setup Google FI from GrapheneOS (at least not from the WORK profile where I had installed Google FI).
... I restored stock on my device - setup Google FI (fast and easy) - THEN when I flashed GOS w/Google FI in my work profile all is working!
HOpe this helps!