I have read on the internet (so it must be true) that Nikon was a little surprised that the Sonnar 105 was becoming popular as a portrait lens and not just a short tele. Perhaps they expected the 85 f/2 to keep that crown. So they improved the 105's near range performance with the Gauss design. The fact that they extracted still more resolution in general use is just the icing on the cake. I've shot both side by side using Tech Pan and the Gauss is a tiny bit sharper (although this might have been just visual due to an increase in contrast) but it took a 32x enlargement to see it and then only barely, in the wings of a distant bird over Long's Peak. I'll print up the neg and post a couple images.
That's pretty much the same reason I read of.
I'll be interested in the images!
...maybe I'll get a Gauss version too.......