Why is ionization energy of indium less than gallium?
The theoretical values of ionisation potential are pretty close to each other.in fact ionisation potential of gallium is almost equal to that of aluminium.Now due to MORE shielding in indium but lack of lanthanoid contraction the affect balances out.As the atomic radii and shielding affect simultaneously takes place from your viewpoint you can claim that the increase in the atomic radii predominates over the zeff.thus indium will have lesser ionisation energy.