With your permission [@Raineh Daze], I could add to Gehrman's backstory that he studied about the Dolls through his connections with Regina's church, but was never accepted as part of the sect due to his occult beliefs. He could have also heard about Regina having been a Doll Hunter, and finding her to be a Doll Master to be a shock to him, and further "proof" to him that Dolls are a corruptive force.