Which measurable factors can affect income equality (Gini index)?

Answer 1

Here is a graph of the gini index vs GDP per capita. All of the developed nations of the world except the US have relativity low Gini indexes, but so do many poor countries
http://www.visualizingeconomics.com/2006/01/04/gdp-per-capital-vs-gini-index/
The gini index correlates with the measures of socio-political instability and negatively with the amount of government foreign aid which can be taken as a measure altruism. In developing countries high economic growth rates correlates with an increases the gini index. Separating cause and effect from correlations is tricky so there are different interpretation of the data.

edit:Globalization, especially trade between rich and poor countries, increases the inequality within countries but decreases the inequality between countries.

Answer 2

Just some examples -

Import
Export
Inflation
Population break-up (urban-rural)
Population break-up (elderly, children, working)
GDP
Corruption (if an index can be developed to measure it), Educational level (again this can be quantified - you just have to rate the higher levels by a scale)
Technology growth (again this can be quantified - you just have to rate the higher levels by a scale)
ICT indicators:
Penetration & spending on:
PC
LAN
Server
Internet