Typically.....around ninety-percent of Germans.....you will never hear the term used....even in a forty-year span. So, it's not a typical word within most circles.
The pure definition? It is the question in a debate which finally gets to the core issue of the whole discussion or argument.
It's supposed to be mythical moment when the guy leading the argument or discussion.....starts to show his agenda to the discussion. A debate master would say that this is the turning point of the discussion.....when he stops the talk and finally notes what the the gretchenfrag of this whole talk is really about.
Original usage? It has to do with some of Goethe's works and this scene where a pact with the devil has been procured and there's some final talk about the whole reason for religion to be in this debate.
Intellectuals in Germany will harp on the usage of the tactic and why gretchenfrage is an important part of any solid debate. With the right gretchenfrage......you can shift a losing debate into a winning debate. At least, that's the idea of the concept.
Generally, my humble advice here.....if you've wandered into some pub and they've reached a point at the pub with several guys arguing and someone throws 'gretchenfrage' out there.....it's probably the right kind of bar for you, and you might want to just settle up and walk another block to a sports bar or old-geezer pub where they play spades.