Greater Spell Focus( Deities and Demigods, p. 50)
The deity chooses a school of magic to which it already has applied the Spell Focus feat. Its magic spells of that school are even more potent than normal.
Spell Focus (PH) ,
Add +4 to the DC for all saving throws against spells from the school of magic the deity selects to focus on. This bonus overlaps (does not stack with) the bonus from Spell Focus.
A deity can gain this feat multiple times, choosing a different school of magic each time.