The headline grabs attention and is a very valid point, but I wish the methodology behind Hillary Clinton's citation of a position paper by the Institute for Women's Policy Research claiming the gender pay gap costs us $447.6 billion in GDP weren't so utterly misleading about the true costs of the gender pay gap.
The position paper lays out an accounting exercise that scales up the earnings losses attributable to unequal pay by gender across the nation, and then expresses that as a share of GDP. There is no scenario under which this is a reasonable claim to make. GDP is the product of our hours worked combined with our ingenuity and our capital formation. It won't change overnight by paying factors more, unless --- critically --- it raises their supplies.
That's the real policy issue, whether women are supplying their full potential to markets for labor and for ideas, or if they are held back by unequal pay, stigma, or other barriers. MIT's Esther Duflo has a nice J Econ Lit piece on these topics, and the subject has been discussed in the development literature for some time.
In probably a sign of its limited impact, I had a tough time tracking down a reference to a 2009 press event featuring Nobelists Elizabeth Blackburn and Carol Greider in which they highlighted barriers to women in developing research careers. Not exactly a pay gap formally, but exactly the kind of thing we should be talking about as a barrier for advancement of female well-being and of national economic well-being more generally.