I could be wrong, but I do think you will get far with Gimp 2.8 From memory (ArtRage?), there are applications which have a 'virtual artists palette' function.
One hurdle is the sRGB colorspace mode see:
https://en.wikipedia.org/wiki/SRGBThis produces that nasty dark border between colours. The development Gimp 2.9 introduces the linear colour space which corrects that problem.
Other than that, Gimp 2.9 has the mypaint brush engine, which helps, there are 'blend' type brushes.

There is also an enhanced smudge tool, an example here:
https://www.gimp-forum.net/Thread-Gimp- ... mudge-toolI think if you want to use Gimp 2.8, you will need to use some other application in addition.