Since it's a container for a very small amount of coal the chance to get Gallant is not very high. However, Dunkirk has Gallant in it and it's possible to get it.

I'd like to add.
This containers main value is collectible items, signals and and camouflages, chance to get Gallant is there, but is quite small, and you should consider it as a bonus, not as the shortest and most convenient way to get that destroyer.
Also, I checked your PT account logs, and there may have been an error in your calculations.