Is the gender pay gap a myth?

My friend said that it was a “myth that has been debunked many times” so it’s a discussion we’re having right now. The pay gap (the idea that women are paid less than men when taking into account all mitigating factors) is a talking point of modern feminism, but does the data support it?

I don’t have an opinion either way yet; I’m just learning about what the data and studies show. But I’m curious to hear your arguments.