Gallery isn't a place, it's an app. It shows all pictures on the phone, regardless of where they get saved to. The only way to keep a picture from showing in Gallery (and that keeps it from showing in all picture apps) is to put a file named .nomedia (with the leading dot) into the folder that the picture is in. Then no pictures in that folder, or any folders in it, or in them, etc., will show. (Or put a dot in front of the name of the picture file, to keep it from showing at all.)