According to a report by USA TODAY, both Wahlberg & Willaims were contractually obligated to do re-shoots based on their original contracts. The reason Wahlberg was able to hold up the production for more money is that his contract had a co-star approval clause — he was therefore exercising his contractual right to deny approving Plummer as a co-star (unless he was paid additional money). Exercising this right was a dick move by Wahlberg but since he had the clause in his contract it makes sense from a logical self-interest perspective. He didn't care about being helpful, he was focused on #1.

Williams saying she would do the re-shoots for free was irrelevant — she was obligated to do them based on her original contract and did not have a co-star approval clause. However based on her statement, I think had she been in the same position as Wahlberg she would have approved Plummer without demanding an additional fee.

Therefore the underlying problem isn't so much the gender pay gap but rather (1) some people are selfish and some people are not and (2) some people have enough clout & sense of entitlement to demand special treatment (e.g. co-star approval, higher salary) and some people do not.

With that said, I would posit that males in our society are more likely to exhibit selfishness, a sense of entitlement, and (by proxy) clout. So if we are serious about addressing the gender pay gap, we really need to address the power pay gap.

In California, it just became illegal for employers to ask you for your salary history. They have to lead negotiations by making an offer relative to market conditions, not your personal market conditions. A step in the right direction, perhaps.