In gimp I can create a black and white layer mask, perhaps with the color threshold tool or with a desaturated luminance mask. And then I can edit that mask with a brush.
I can do most of that with Darktable but editing the mask is often not so easy. I can flip polarities, combine drawn and parametric masks and usually get what I want. But I think it could be easier.
If Darktable had a yellow-colored brush tool that could be feathered and/or blurred you could add to an existing parametric mask. A corresponding erase tool that could also be feathered and blurred would be the cat’s meow.