A gain of $15b? That's roughly the value of 100 metric tons of gold, remarkably close to the 129 tons that the article says was moved... did they double the value of their gold?
That's an orthogonal matter (if the gain/loss was calculated correctly).
But they didn't just move gold bars around, is my point, and in what they did (sold, rebuy) there indeed was an opportunity to make a gain.
When something is "realized" is a matter of accounting. It means to make the change, they sold the gold fo currrency, then bought it back. For many of us, realizing a gain is when taxes happen, though I'm not sure what it means for a nation state.
https://www.investopedia.com/terms/r/realizedprofit.asp