Perhaps Reinhardt and Gregor became Templar Knights at the same time, rivals perhaps, with yin and yang methods with which they handled situations. Maybe when Deacon left, Gregor felt as though he abandoned the Order. Just a thought. Deacon's CS also says that he has helped apostates escape the chantry, so at the very least, we could make Gregor aware of that, either through rumors or eyewitnesses.
"Stick around... I'm full of bad ideas." -Isaac Clarke
"You've chosen your Answer. Now see it through." -Serene Haze
"And I heard a voice in the midst of the four beasts, and I looked and behold: a pale horse. And his name, that sat on him, was Death. And Hell followed with him." -Johnny Cash
Check out my literary comic: Kurosai Chronicles