In S9:E15 of The Walking Dead, "The Calm Before", at around 13 minutes in, the leaders of the soon-to-be Coalition decide to sign the charter originally drawn up by Michonne as part of a mutual defense pact against The Whisperers.
The King is the first to sign it (as "King Ezekiel") and then Carol, who signs it as "Queen Carol". Even though she has refused many times to use the honorific, this is AFAIK, the first time she actually accepts it, which shows the formality and momentousness of the occasion.
When it is Alexandria's turn to sign, Michonne tells Gabriel that he, as the head of the Council, should sign - not her. He does so, but we see him sign just "Gabriel" before the camera cuts away (he may or may not have included his last name).
However, he is still a practicing Episcopal priest and he still wears his collar. If the formality of the situation is such that Carol uses her title of Queen, shouldn't he use the honorific he has already used for years and sign it as "Father Gabriel"?