Bank of France "transported" their reserve by selling the gold held in New York, and subsequently buying the same amount in European market.
They opted to do so because it's just more efficient. It takes a lot of efforts to physically move 129 tonnes of gold after all. And as a side effect of this relocation project, they ended up recording a capital gain. It's nothing-burger.
The transport would likely be quite expensive as well. Lots of armed people needed to move gold around, plus special vehicles.
For context, in 2025H1, 480 tons where moved from CH to the US (I assume originating from UK after being recast).
My guess is that the choice to sell rather than transport was also due to using the (at the time) price divergence between US and European markets. (arbitrage + not having to pay transport + refining)