This is not gain at all. At least in theory: You own some tons of gold at the start of the process, you have the same tons of gold at the end of the process.

The only real gain is that you have gold in the US custody and the US can be tempted to just use it without telling you anything.

In other words, you had "paper gold" or "virtual gold" that the US can confiscate anytime, for example after invading Greenland, blackmailing France to do nothing.

You gain custody of what is yours.

which can be the difference between losing that entire amount or gaining it, and in this situation with this America, this is a big win if they manage to get it back in fact, if it hasn't been stolen or sold already

From the full press release:

"In 2025 and at the start of 2026, while the volume of gold reserves remained
unchanged, the Banque de France had to align a residual portion (5%) with technical guidelines, resulting in a significant realised currency gain. This exceptional foreign exchange income totalled EUR 11 billion for 2025."

-- the keyword here likely being "realized"

Is the logic that it's "unrealised" while the gold is stored in the US but becomes "realised" once it is stored in Paris? Why?

You bought it once at X price, it's realized when you sell it, it's unrealized while "open"

If they held it for 100 years and finally sold it, then profit/loss is realized now

But then they bought it again. They had 129 tons of gold, and now they still have 129 tons of gold. Where does the realised gains come from?
They "realized" it just for a short time.

It's more of a loss for the USA, which IMO is the unwritten point of the article.

France upgraded their gold bars to a new standard and as they were doing that, gold has appreciated massively in price, so France has the new shiny easier to trade bars, and the USA has the old harder to trade bars.

They can be melted and brought to the modern standard, which is what they did with the rest of their holdings on the old continent. They sold these only because it was cheaper than transporting it.

As @somenameforme wrote:

[] they sold their 'non-standard' (seems to be bars below the modern purity standards) US reserves, and replaced them with new reserves purchased elsewhere which are now stored in France. As the price of gold continued to rise as they did this, they ended up making a bunch of dinero while also centralizing their reserves.

sounds like a gain to me.

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?

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

So they could sell it again and buy it again and realise another $15b?

Assets like this are one of the complexities in calculating national import and export figures.

For example, imagine there's some German-owned gold in a UK bank vault, the owners sell it to a UK broker who sells it to a Chinese investor? The physical bars don't move, but on paper it's been imported to the UK then exported.

But a lot of people looking at export figures are expecting to learn things about the manufacturing industry, and picturing exports as washing machines, cars and computer chips - which imply lots of well paid jobs for skilled labour. So the UK reports import/export figures with 'non-monetary gold' listed separately.

(The fact flows of gold are highly volatile allows a classic bit of political sleight-of-hand - if you include gold, UK exports are both up and down since Brexit, depending on the pair of dates you choose)