I took Victor‘s advice and went with a “less (color management) is more” approach, by turning off the color management mode in GIMP. (For GIMP 2.8 in OS X, go to the menu Gimp –> Preferences –> Color Management (in the sidebar) –> Mode of Operation –> select “No color management.”) I then discovered rather serendipitously through my troubleshooting internet searching that Nikon has a new photo management/NEF (RAW) editor — well, new to me — and it is called Nikon Capture NX-D. Well, hot dang. I downloaded that toute de suite, installed and what do you know? My photos now look the same after I convert them to Tiff files, which was not the case when I was using Nikon View NX2. Will wonders never cease. All of which reminds me, now that my blood pressure is down again, that I need to uninstall some software.