The GINI index estimates we publish are computed based on income or consumption data measured from household surveys; not based on sub-national GDP data. Access http://iresearch.worldbank.org/PovcalNet/index.htm for more details about our methods, to replicate our results and to access the underlying data. Note this is clearly documented in the WDI print edition, but not all that metadata makes it to our on-line database nor to our API. But to get more information, click on "Notes" icon while using the DataBank data retrieval system accessible from http://databank.worldbank.org/ddp/home.do?Step=2&id=4&hActiveDimensionId=WDI_Series. Hope this helps.