Well an upgrade. I was asked to demonstrate free software at the local club in Mold Flintshire. I found Rawtherapee very good and Gimp had also moved on with it's own format saving layers and masks.

DNG I would use with Pentax but with Nikon no way. The Pentax uses 14 bit RAW files but Nikon uses 14 bit so moving to DNG means losing two bits.

As to apply with Gimp not really required the add mask gives a "Greyscale copy of layer" which does the same thing.