Since you'd have to open it from the Preview after loading a G-code file, I think something that is basically the FFF settings window with all the setting controls "grayed out" (disabled) would be sufficient. I suppose you could also put " - [Read Only]" in the title bar.
I'm trying to imagine what Jeff Johnson would have suggested in his "GUI Bloopers" book. Great reading if you're developing a user interface, by the way. He does a great job of showing what "grayed out" should actually look like -- compared to how some software companies have implemented it!!!