Well, eyeballing it might easily be as accurate as most game mechanical approaches.
What I kind of would like now is maybe something akin to the GURPS CR system, with “equality” scores from 0 to 6, and sketches of the resulting societies. So for example ER 4 would be a country like the United States. Of course the extremes would be more Platonic ideals than actual social patterns: a godking served by a vast army of destitute servants, or a hivelike communism, maybe. (Though you could turn the first into the second very easily by removing the godking, or replacing them with the collective personality of The State or The People.)
Addendum: Dividing the percentages up at 91.67, 75.00, 58.33, 41.67, 25.00, and 8.33, I see that the world basically splits up into three groups: Southern Africa, the less developed world, and the more developed world. Iceland and Ukraine almost make it over the edge into a fourth group. On the other hand, the US Census Bureau gives the US a much higher Gini, with only two states falling into what I’m calling the “developed countries” range: Utah and Alaska. It’s curious that the World Bank and Census Bureau estimates are so different; I wonder how their methodologies differ?