The gender gap in political knowledge is a classical problem of Western democracies. In the 21st century, political knowledge is still unequally distributed between men and women, as many cross-section studies have shown. This is an indicator of women's disempowerment and the distance which remains to be covered to achieve an inclusive and sustainable society. Could public policies and gender equality laws change the situation? Using a longitudinal database in which 600,000 survey responses are analysed from 1996 to 2017, this case study of Spain aims to shed some light on this question. It combines sociological and political approaches in line with the development theory of the gender gap of Inglehart and Norris (2000, 2003), whose core argument is that modernization changes cultural attitudes toward gender equality. From this perspective, this paper proposes the following hypothesis: the modernization process of Spain (from a dictatorship to a democracy) has given rise to changes in traditional sex roles, driving women's access to political knowledge and diminishing the gender gap. This is a step towards achieving objective number 5 of the 2030 United Nations Agenda for Sustainable Development (gender equality and empowerment of women and girls), according to which gender equality is not only a fundamental human right but a necessary foundation for a peaceful, prosperous and sustainable world.