The US gold would have been on the books at the original purchase price, so something like US$35 from 1910 (when a penny had a purchasing power of 38 cents now). Having deemed it more efficient to sell that gold and buy the same amount to replace it, the new gold is on the books at the 2026 purchase price. As the 2026 money price is far higher than the 1910 price, the value on the books shows a dramatic realized capital gain.

No gain would have shown for the gold that was simply moved, even though in this case the buying and selling was simply a more efficient way of doing the equivalent of moving the gold.

Gold that was simply moved wouldn't show the same gain.