In the UK, that is already in progress - https://www.theguardian.com/society/2017/apr/06/gender-pay-g...
>The gender pay gap is the difference between the average earnings of men and women, expressed relative to men’s earnings. For example, ‘women earn 15% less than men per hour’.
But this is not what should be measured. If there's a mining company that employs 90% men working down in the mine and 10% women doing accountancy, then of course the average wages of men are going to be different than those of women, but that tells us nothing about equal and fair treatment of men and women at this company.
Lies and statistics.
It's a perfectly reasonable thing to measure as long as it's not being used as a proxy for equal pay (which it explicitly isn't in this case).
What it can show is systemic bias. Now, unlike unequal pay, it's not necessarily blatantly unfair bias. It might not be unfair at all depending on the detail.
But knowing that there's a gap and understanding the reasons for the gap can be beneficial.