Hi Mike,
I presume you are the Mike I have communicated with before from Florida?
If I want to work an image in GIMP and how I was planning on doing it. D Darktable I use the open feature in GIMP and being a RAW file it opens the image in DT. I do the necessary edits in DT and then just shutdown DT and the image is moved automatically across to GIMP as a 32 bit floating point file. I then edit the image in GIMP and export as normal. The other way I use the two programs together is to edit in DT first, export as a 16 bit tiff and then open the 16 bit tiff in GIMP. I appreciate that these may not be the solutions you are looking for but it works well for my needs.