The 'wage gap' actually mainly exists because women work less hours per year (up to ~20% less), and because they they work in different industries, and on the whole do different jobs. There's no discrimination.
Or rather that the discrimination is cultural and leads to life choices that make the wage gap structural.
For example: We live in a culture that devalues young girls who excel in math, leading to lower achievement in this area[1]. By the time they go to college they choose differently than men and "work in different industries"
[1] http://www.jstor.org/discover/10.2307/2096370?uid=3739856&ui... for one example
That is a fairly blanket statement that could use some backing up.