At first, you should pay much more attention to your Model's structure. It means we must be careful about our economy environment.
By doing that, you will become more aware about;
What regressors could/should be piked?
What kind of those must be introduced in regression?
If you go through these steps you will be able to pick the most appropriate variables.
If your are interested in the statistic relationship between income inequality and economic growth, you should use GDP per capita growth since the others indicate level rather than growth.
More generally, the choice of the variable in your (and any other) regression depends on your economics model, otherwise you are just doing some linear projections.
A good ...