It's certainly true that there are, if you like, two gender pay gaps, and this set of statistics is, at least mainly, referring to the second kind.
I don't think there can be any argument any more that two people working the same job with the same hours should be given the same salary, and that is the (illegal) first kind of "gender pay gap". But the second kind is still important to understand because it exposes the divides in representation in any given job, and then -- which is, again, the only point I am trying to make here -- that raises questions about why these divides exist.
As a technical term, then, this is *still* a gender pay gap. If you can take a given woman and be reasonably confident that, on average, she will earn only something like 80%* of any given man, then that's a gender pay gap. It may arise because of multiple issues -- such as maternity leave impacting job prospects and promotion chances -- but regardless of the reasons, it's important to understand them. That way, if there are not justifiable, then they can be properly addressed.
*This figure is about the right unadjusted gender pay gap for the US; I don't have the UK figure to hand right now, although I think the pay gap in the same metric is rather smaller over here.