Is a GS really that close to a comparably equipped A6? I can see where that would be a marketing problem regardless of their comparative merits. Even a base GS comes very well equipped, but I haven't looked at Audi pricing lately - might be better than I am imagining.
"Very well equipped" has a different meaning in this segment.
In any case a full load GS is about $70k, roughly equivalent A6 3.0T is $76.5k. Admittedly I skipped the ridiculously priced $6.5k B+O sound system on the A6 so that would swing things, but honestly not many people would consider audio to be the swing factor in these two cars.
At that price the Audi has several advantages that I don't believe the GS can match at any price. Example full speed range ACC, adaptive LED headlamps (I think), massage seats, 4 way lumbar, 360 deg camera, and of course the ZF 8 speed. If reliability were not a concern I'd buy the A6 100% of the time in the above comparison.