You’re welcome, although I’ve seen comments on here that indicate that at least a few others are doing something similar, i.e. using layers in GIMP to do things like WB and/or exposure blending.
So far it’s been my primary way of recovering highlights, reducing blown-out lights, and raising darker parts of images.
@david is hinting at similar things e.g. here:
Although I can hardly call my process a “technique”. Or a process. It’s more like (barely) educated guesses as to the effects of a particular layer and mask and then fiddling around with it, inching ever closer to something I like well enough.
I’m neither proficient nor efficient at it is what I’m saying 
I’ve also redone my edit of your image, softening the gradient to something a little more plausible:
