Sure, the pay gap DID exist, but after extensive studies by unbiased third parties to actually find out either the cause of the pay gap, or to disprove it altogether, it was found that in many instances in our modern world, the pay gap does not exist, and that women actually are paid less on some occasions because they work less, take more time off, aren't paid a maternity leave when they have children, etc. This all affects overall income earned. Women on average pursue different career goals, where as men want to be top earner, or make partner, or excel at a quicker rate than others around them by staying later, working more, not taking time off, working weekends, etc.
By any legal sense, though... the pay gap would be illegal if a male and female with the same level of education, experience, seniority, qualifications, hours, and job position received different pay. That simply is not the case, and if it could be done by major corporations, men would never be hired, since they would know they could pay women less to do the same job.
Now back in 1945, you could get away with that sort of thing, just like you could get away with paying black employees less, or hispanics less. In some ways that still does occur, but mostly in a undocumented worker setting where these people are paid less because they are being paid under the table and they can't rely on the Department of Labor to step in on their behalf.