This myth is based on a priest in the game, also believed to be a reaper. A reaper, in basic terminology, is a person that reaps (cuts and gathers) crops at harvest. Reapers also serve as an allegory for death in various cultures. This idea that the priest is a harbinger of death instead of peace, is common in the GTA myth community.
In the mission Madd Dogg, Madd Dogg wants to commit suicide. At the bottom of the ledge he plans to jump from, there is a priest encouraging him to jump, rather than helping him.
The priest also appears at funerals. Several myth hunters theorize that he intentionally fakes his grief. This myth is most likely not authentic, but keeping in view the origin of the myth, it's widely considered as a hoax.