There is both a pre-tax and a post-tax GINI co-efficient - it's not fundamentally part of the co-efficient.
This is further complicated by taxing at source (e.g. if your employer had to pay tax prior to giving you your pay, that likely wouldn't be counted).
Different data sources use either the pre-tax or post-tax figures.
See Page 26 of http://documents1.worldbank.org/curated/en/59073146799250871...
