I firmly believe that Gods as ethereal beings don't/can't have gender. (Gender is limited to the physical forms that they use, and disgard, while visiting the mortal realms)
My favorite theory of how gods get their physical forms is through the imaginations of their worshipers (if their followers imagine a Masculine Demon with big sharp teeth & tentacles while they pray then their god will draw on that when creating their form), so with the MC as Nergals chosen one "he" is probably doomed to being a "she"... more specifically a Big Titted Bimbo she
, regardless to what gender "he" was to previous followers... It's not like there is a statue to shape the MC's imagination like there is for Qetesh