Hi Peter,
Best is to be pragmatic. Do your initial things in RawTherapee, like demosaicing, perhaps denoising, perhaps other stuff, then send the file as 16-bit to an image processor like Gimp >2.8 where you apply your gradients and/or other things, then export as 16-bit and go back to your image processor of choice to do the rest of the work.
Nothing is perfect in this world but by combining several workflows one might pretend otherwise…
Regards.