I can speak to that. One factor that's important to consider, when considering occupation and the gender wage gap, is the high proportion of women in public occupations, government sector occupations. Of course, these occupations are also mostly unionized. There are studies on the gender wage gap that explain the impact of unionization on the gender wage gap, and I believe it is an important factor.
It would probably be prudent, rather than saying how much more important I think it is, to point you to the research.