There was a gender pay gap when the work required physical strength. This is because men are, on average, physically stronger than women. They are more capable of hauling a plough or heaving a sack of coal. When work meant physical labour for the most part, men were economically worth more. They were not intrinsically worth more, it was just that, on average, their labour could add more value than that of a woman. They were paid higher wages because of this. As physical labour has been made easier by machines, and desk jobs and service industries have become more significant employers than heavy industry, the labour of women has been more equal to that of men, and their pay has risen accordingly. In Britain today there is no significant gender pay gap. Women in their 20s earn a little more than their male counterparts.
There is a pay gap as they grow older, but this is a maternity pay gap, not a gender pay gap. Women who take time out of their careers to have and raise children earn less over the years than those who do not. This is for most of them an option they have chosen to exercise. Most do it because they want to, trading the higher salary that might otherwise result for the greater satisfaction and happiness engendered by starting a family. As they take time out of work, they mount the promotion ladder more slowly than their counterparts who make uninterrupted progress.
It is very important when looking at the statistics on this to compare like with like, that is to compare full-time employment with full-time employment. Some women prefer part time jobs because they offer better opportunities to achieve the balance between work and family that they seek. Part time jobs tend to pay less than full time employment, creating the erroneous impression that women are being paid less for the same type of work and the same amount of it. They have chosen a lifestyle that pays less because they prefer to have children be a part of it.