By the power of truth, I, while living, have conquered the universe.
Posted by: LEGEND ST
It is speculated the Precursor becomes the Gravemind. But still somewhat unconfirmed.
Rather than to say he IS the Gravemind, would make more sense that what we know about Graveminds now (They are the culmination of the minds absorbed by the flood that take form after enough biomass has been collected), and combine it with what we know about the Precursor, that he is EXTREMELY ancient, and likely the most powerful, knowledgeable mind in the universe as-per his release.
What this would mean together, is that the Gravemind isn't the same being as the Precursor, but that the Flood defeats the precursor, and he becomes the most prominent mind in the Flood's being.
We have never seen a fully formed Gravemind beyond the one in the trilogy. The others (Halo Wars and Halo CE) were just Proto-Graveminds, and they were just massive blobs with no defined appearance.
In Human Weakness, the Gravemind states he speaks in trochaic heptameter by choice because he has absorbed the minds of poets from thousands of other races. He states in Halo 3 that he has destroyed "fleets of thousands", and "consumed a galaxy of flesh, and mind, and bone".
None of this would make sense if he wasn't the Prisoner, since the first time he's released that we see is at the end of Halo 2 where he hitches aboard In Amber Clad and goes to High Charity. None of his dialogue would make sense if he was a recently formed entity, stuff like "I am a monument to all your sins" would make no sense at all. But apply his dialogue to the actions of the Prisoner and it makes perfect sense.
There is one issue with the connection between Mendicant Bias and Installation 05. Mendicant likely didn't control 05, if you remember, mendicant could only control 5 of the Halo rings, while the Forerunners maintained control of exactly 7 (The amount that existed in the galaxy at the start of Halo CE). Though I suppose the 6th ring Bias tried to control, but "couldn't" could easily have been 05, and Bias could have purposely been delivering the Ring to them.
Mendicant was given control of Installation 05 to oversee the testing of the Flood - his serial number is 05-032. This would be the Ring he stole originally, fired at Charum Hakkor and had the Prisoner stow away on.