I haven't driven the 6GC but I do have a 6. I have driven a 7 series and the 5 series. All the 6 series are more like a two door 7 and not a 5. The ride, features and attention to detail are all like the 7. I would say the 6GC is a sleeker sexier 7.
If you have the time / opportunity spend some time in a 7, then spend some time in a 5; then drive a 6. There is a big difference in the execution of luxury. The 5 is nice but there is a reason it starts off so low in price.
As for the S7, I would have to sit in it and drive it. Overall it's a great car from what I see.
I almost got a CLS, nice car lots of features for the price. It seems to try very hard to be special, kinda like a woman with too much makeup.
I had a E92 M3 loaded, loved that car. But since it's driven on the street 99% of the time she never really had a chance to shine. So I wanted a car the was great on the street and fun enough to drive.
I love my 6 and I love it even more with my Dinan Stage 2