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.

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"

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