Board Thread:Editing Discussions/@comment-6697950-20150620053117/@comment-6697950-20150621135025

The chōonpu is a just prolonged vowel mark for katakana. It just so happens that the "r" in English frequently serves a similar function. Underworld names aren't supposed to be English, so there's no telling what the official spelling of the name would be.

Anyway, the ei sound in Japanese phonetics is basically the same as a long Japanese "e" sound, so both ゲール and ゲイル are prerty much just alternative variants.