I've tried reading a few things now, and I think that is going to be your best starting point. I can only get so much without knowing the French words, since I'm sure the detail of the translation is much more important when talking about something as specific as tiny structures and textures.

It sounds like kosher salt is more flakey, while gros sel is cubic, but I'm thinking that they are not fine salts and will leave more air pockets when measuring by volume should keep you closest. Grey salt from the sea is going to have some taste that a white salt from a mine isn't, but if it's just being used 90% for whatever osmosis trick it is doing to potatoes, that probably isn't going to make a huge difference.

My understanding is all recipes in English (or at least from America) are written with kosher salt in mind. Salt that would go in a shaker is much too fine and will make food much saltier than intended, it's probably twice as much salt.

It's easy to slip one's mind trying to share something like this, that seems like it would be the most mundane recipe, as probably for both of us, salt and potatoes are daily staples we don't pay much mind to, we just know what to use.