User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-93.132.51.100-20150914122042/@comment-24834479-20150914234432

Castiel possessed his vessel when Jimmy Novak was still inside it. I suppose the if the soulless body said yes to the Angel, it could still inhabit it.

Gabriel did not actually create human beings. They may or may not have had their own consciousness, but they were just part of his sand box illusion.

If Castiel left his vessel, then it would wander as a soulless body.