It is tripe that comes from thickest parts of the first stomach (called the “rumen” in English, “panse” in French) of the animal. It is considered the least desirable tripe amongst tripe fans. The thickness will vary, and it may have a layer of fat attached that needs to be removed.
“Gras-Double” is a misleading term to anyone who knows some French, as the word “gras” usually indicates “fat..” Here, though, it doesn’t mean “double fat”, just the “fattest, i.e. thickest part of the “panse.”
In English, it is also called plain tripe, smooth tripe, blanket tripe or flat tripe.
When trimmed of its interior membranes, the French call it “boeuf blanc” (“white beef .”)
Does not need precooking before being cooked.