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?
If you buy something for $10 and sell at $15, you realized a gain of $5. If you then buy at $15 and sell it at $15, you realized a gain of $0.
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.