This whole process infuriates me. The gender wage gap is an empirical question. It exists. If we believe it's a problem (I do; you may not), it has the easiest solution in the world: give women more money. Seriously, I just solved the gender pay gap, right there: give women more money.
There's all kind of ways you could do that (wage mandates, tax credits, etc.) and they all have pros and cons, but searching for proximate and ultimate causes here is kind of stupid. It exists; if you think it's a problem, the solution is breathtakingly obvious.