Perhaps a lever could be added near the gondola that, when pulled, makes it so that it closes the game after a fade to black once you enter the gondola?
Above is my suggestion for how to do it, but there could of course be others. But, unless I'm mistaken, you currently just have to close the game via external means, so a way to do so that is integrated into the game world like the various menus are might be nice, at least in my opinion.