This is an easy pledge to make, if you pay people based on their education and actual years of experience. Why? Because - if you look at it that way - there is no gender pay gap.
All the studies that show a substantial gender pay gap either (a) equate different professions, or (b) compare people based on their ages.
The first of those is obviously flawed, because different professions are, in fact, different. This includes studies that compare average pay in an entire region, because women and men do tend to congregate in different professions. Exactly why women go into lower paying professions is a complex issue, but really, it doesn't matter. All that's important is that women and men both have the choice to do what they like. If a woman wants to become a civil engineer, or a man wants to work in a kindergarden, those doors should be open. If they are, then there is no problem: people can choose a career that suits them.
It's the second type of study that's more insidious: comparing earnings based on age. More women than men take time off, or work part-time, to raise children. Hence, the average (to pick an age) 40 year old woman will have less experience than the average 40 year old man. Some opportunities may be entirely lost: for example, not being available for a high-intensity or high-travel position may make one ineligible for a later promotion. An alternate approach is to compare women with and without children. What a surprise: women without children earn much more than women with children (article is in German).
So it's easy for companies to pledge to pay women the same as men - because they already do.