I *think* I figured out the code to get the Oracle to work correctly, though I still need to test it in-game, which may take some time.

EDIT: Well, it caused my game to not start up correctly, so I'm guessing I messed up somehow. Not sure which part of my code was the issue, but it's non-functional for now. If anyone wants to take a look, let me know what you find out!

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The Oracle uses a 'SubjectRequirementSetId' to determine which districts are in a city so it knows to boost their GPP. Every other district is defined in (base game) Leaders.xml or Expansion1_Leaders.xml, but I have not found if/where there's a spot where 'District_Is_Government' has been defined, so I had to reverse-engineer that section of code (the part in <GameInfo>).

Apart from that, this is basically copy-pasted from the (confirmed working) 'Tweaks for JFD's Great Theologian' mod. Insert these lines into the GreatSovereigns_Core.xml, at the bottom after </Building_GreatPersonPoints>. The code below includes both </Building_GreatPersonPoints> and </GameData> so you can remove both lines.

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</Building_GreatPersonPoints>
<Modifiers>
<Row>
<ModifierId>ORACLE_GREATSOVEREIGNPOINTS</ModifierId>
<ModifierType>MODIFIER_SINGLE_CITY_DISTRICTS_ADJUST_GREAT_PERSON_POINTS</ModifierType>
<SubjectRequirementSetId>DISTRICT_IS_GOVERNMENT</SubjectRequirementSetId>
</Row>
</Modifiers>
<ModifierArguments>
<Row>
<ModifierId>ORACLE_GREATSOVEREIGNPOINTS</ModifierId>
<Name>GreatPersonClassType</Name>
<Value>GREAT_PERSON_CLASS_GreatSovereigns</Value>
</Row>
<Row>
<ModifierId>ORACLE_GREATSOVEREIGNPOINTS</ModifierId>
<Name>Amount</Name>
<Value>2</Value>
</Row>
</ModifierArguments>
<BuildingModifiers>
<Row>
<BuildingType>BUILDING_ORACLE</BuildingType>
<ModifierId>ORACLE_GREATSOVEREIGNPOINTS</ModifierId>
</Row>
</BuildingModifiers>
</GameData>
<GameInfo>
<RequirementSets>
<Row>
<RequirementSetId>DISTRICT_IS_GOVERNMENT</RequirementSetId>
<RequirementSetType>REQUIREMENTSET_TEST_ALL</RequirementSetType>
</Row>
</RequirementSets>
<RequirementSetRequirements>
<Row>
<RequirementSetId>DISTRICT_IS_GOVERNMENT</RequirementSetId>
<RequirementId>REQUIRES_DISTRICT_IS_GOVERNMENT</RequirementId>
</Row>
</RequirementSetRequirements>
<Requirements>
<Row>
<RequirementId>REQUIRES_DISTRICT_IS_GOVERNMENT</RequirementId>
<RequirementType>REQUIREMENT_DISTRICT_TYPE_MATCHES</RequirementType>
</Row>
</Requirements>
<RequirementArguments>
<Row>
<RequirementId>REQUIRES_DISTRICT_IS_GOVERNMENT</RequirementId>
<Name>DistrictType</Name>
<Value>DISTRICT_GOVERNMENT</Value>
</Row>
</RequirementArguments>
</GameInfo>