The file that you're looking for is ..\Path\To\Sid Meier's Civilization V\Assets\DLC\Expansion\Gameplay\XML\Civilizations\CIV5Civilizations.xml. You can search for "Civilization_Religions", which should bring you to line #4902. You can find the G&K civilizations in CIV5Civilizations_Expansion.xml, starting at line #414.
Christianity: America, Aztec, England, France, Germany, Greece, Iroquois, Rome, Russia, Austria, Byzantium, Celts, Ethiopia, Maya, Netherlands, Sweden, Spain
Islam: Arabia, Egypt, Ottoman, Songhai, Carthage
Taoism: China
Hinduism: India
Shinto: Japan
Zoroastrianism: Persia
Buddhism: Siam
Tengriism: Huns
The general rule is that the preferred religion is the one that was most prevalent or most official during the leader's reign or, if that religion is not implemented, then the one that is most common today.