If anyone wants to have a go I have posted my first draft of Bitmap to Gradient - Create Linear gradients from bitmap files and existing embedded images

▶ Creates a simple Linear or Radial Gradient
  from a bitmap file or existing embedded image
▶ Appears in 'Extensions>Images
▶ Would recommend using this on a blank document
  and copy / pasting the resulting objects to the
  document you are working on.

https://gitlab.com/inklinea/bitmap-to-gradient

I've made a slight update, to incorporate some optimisation ( removing runs of duplicate stops )