Gibbon spells it as Elagabalus too.

Gibbon was probably the primary source for much of HoR

Though definitely a major influence, Duncan describes reading through the work as a child, I don't think it was his primary source. He had learned enough about history to doubt Gibbon, and tried to read available primary sources as much as he could.

> Elagabalus's family held hereditary rights to the priesthood of the sun god Elagabal [...] The god was later imported to Rome and assimilated with the sun god [...] In Greek, the sun god is Helios, hence Elagabal was later known as "Heliogabalus", a hybrid of "Helios" and "Elagabalus".