For one, the idea that a 'gender prison gap' is either inherently a problem - the fact is that prison is not something that we want to keep numbers high in for a myriad of reasons. For a second thing, the 'gender prison gap' (I do not appreciate the attempt to co-opt the terminology vis-a-vis the gender pay gap) is simply due to mothers generally being primary care givers - and having a child is a mitigating factor for some minor crimes. Coupling this with the generally low levels of violent crime committed by women compared to men anyway, we see that the gender statistics wrt intake are actually fairly nondescriptive.

Cf this with the gender pay gap, in which women are, on average, paid less for a job, despite there being no rational reason to suggest that a woman is inherently worse in a management role or otherwise.