I suspect that the "wage gap" is primarily a creation of comparing non-identical work.  To determine whether there is a real wage gap, double blind studies where researchers see job descriptions, separate them into equal work categories without knowing the gender of the employee and then only after gender free identification of job categories, employee tenure, and employee experience has been completed, reveal the gender of the employees.  Double blind studies are often utilized in order to eliminate bias in those who perform the study.  It creeps in in many ways.