Erwan
50 Cal.
Right...The translation appears to be (surprise!) "cartridge" and at least quasi-synonymous with "cartouche", although it appears that "gargousse" is more specifically applied to cannon where in English it would mean something like "load of gunpowder in an envelope (or bag)" -- or something like that. Such as "packaged charged".
A "gargousse" is a packaged powder charge, as well for a marine gun as for a land gun and even some weapons like Sharps...
The gargousse is inserted through the breech and, once the falling block is in place, the bullet is loaded through the muzzle. The loading is longer and more tedious but it allows to use important powder loads without having to pour the powder in bulk in the chamber...
We can also note that this method is also used by some with the 45-70-500 and even the 45-120-500 to have the case completely full and the bullet on top...