This paper proposes to measure and to evaluate gender gaps and gender inequalities by means of the decomposition of an inequality measure. A three-terms decomposition of the Gini index is applied, thus allowing to take into account also the role of overlapping between female and male subpopulations. We develop an unified framework for the evaluation of gender gap, linking traditional measures, based on subgroups income means, to the approach related to inequality decomposition, and showing how overlapping component represents a key issue in gender gap analysis. An analysis of the income distribution of the Italian households shows how gender gaps represent a major source of inequality, without particular improvements during the last 20 years.