The term "gender gap" refers to the observed inequity in earnings, whereby men earn significantly more than women both on average and when performing the same job.
There are a ton of sexist rationalisations for why it is right and proper that the gender gap should continue to exist.
Let a minor Greek Goddess explain why those rationalisations are made and why they are wrong: Echidne of the Snakes did an excellent 3-part series on the Gender Gap last year (1. Theory, 2. Empirical Evidence and 3. Addressing the Wingnuts).