Not comparable numbers
It's an apples to oranges comparison.
Gal Gadot's $300,000 was her base salary for each of Wonder Woman, Batman vs. Superman, and the Justice League. Her salary would also include performance incentives. We don't know what those were.
Henry Cavill's $14 million for Man of Steel includes such performance incentives.
Example source, Vanity Fair:
As the Elle article that sent the stat viral said itself, Cavill’s $14 million earnings include bonuses for box-office performance, while Gadot’s $300,000, per a 2014 Variety report, is just the base salary for each movie she’s made thus far in the DC Universe.
which goes on to say
...Robert Downey Jr., Chris Evans, and Chris Hemsworth, all of whom were reportedly paid less than $500,000 for their first solo superhero outings but eventually landed much bigger paydays for subsequent entries.
So we won't know if Gadot is making less money than Cavill until after her performance incentives are calculated. Beyond that, we won't be able to compare her to people like Downey ($500,000), Evans ($300,000), and Hemsworth ($150,000) until she signs a contract for a sequel.
Questions about Cavill's income
There are also questions about the source for Cavill's payments. For example, Vox says:
And after people pointed out to Duca that the $14 million figure had come from a questionable source
For example, if we look at the original source, Elle, it now says,
Update: Reports that Henry Cavill earned $14 m for Man of Steel are unconfirmed. Although the pay gap persists in Hollywood, this example is not adequately supported by the information available.
So even the people who released the initial story are unwilling to stand behind it.
TL;DR: We don't know if the numbers are real, but even if they were, they are not comparable. We should either compare both base salaries or both after performance incentives are included.