What does the Gini index mean in a biochemical context?
The paper by Graczyk (2007) is probably relevant for you. It says that the Gini index is a measure of reactive selectivity of kinases, with values close to zero indicating no selectivity and values close to one indicating high selectivity, and it is created in direct parallel to the Gini index in economics, which is used to describe economic inequality. In its basic form, the Gini coefficient is a measure of statistical disperson. Also see Anastassiadis et al. (2011) for a more recent example where the index is used.


  Abstract
  A novel application of the Gini coefficient for expressing selectivity of kinase inhibitors against a panel of kinases is proposed. This has been illustrated using single-point inhibition data for 40 commercially available kinase inhibitors screened against 85 kinases. Nonselective inhibitors are characterized by Gini values close to zero (Staurosporine, Gini 0.150). Highly selective compounds exhibit Gini values close to 1 (PD184352 Gini 0.905). The relative selectivity of inhibitors does not depend on the ATP concentration.


References:

Graczyk. 2007. Gini coefficient: a new way to express selectivity of kinase inhibitors against a family of kinases. J Med Chem. 50(23) 

Anastassiadis et al. 2011. Comprehensive assay of kinase catalytic activity reveals features of kinase inhibitor selectivity. Nature Biotechnology 29: 1039–1045