You're welcome, David! French gray is the finish obtained by removing the colored portion left at the surface by the black oxide (hot salt bluing) process. The salts used in this process form a micro-thin layer of hard, black rust at the surface when reacting in the presence of oxygen. The "bluing" penetrates into the surface just a bit beyond the color, though. We can remove the colored portion with the application of a weak acid solution (like phosphoric acid, which most people know as the acid found in Coca-Cola and toilet bowl cleaners) and retain the nearly colorless substrate immediately beneath it. This gray substrate offers nearly the same corrosion resistance as the original finish, which is why french grayed guns resist rust so much better than naked steel. At least that's how it was explained to me, and I got the info second hand from Lynton McKenzie.

There has been plenty of discussion on this topic on the forum here over the past year. A search on it will likely give you all the info you need.

I'm still convinced, in my mind, that all the 'polishing' done to a gun prior to case coloring (such as when a gun is re-colored) is being done all wrong.

There's no reason a gun couldn't be prepared in a manner that does not degrade the engraving. It would take some trial and error but the right process of abrasive media blasting would do the trick.

I had a 16Vh that had been hot blued, I took some apple cider vinegar
put on a soft rag and took the hot blue off and the gun looked great.