(At this point I personally pretty much just always use G’MIC’s Smooth [bilateral] - this produces the best results by far).
For the Gaussian Blurs, I’ve had good luck with radius values around 20% - 30% of an image dimension.
The only extra thing you’ll need to be able to replicate this effect is G’MIC for GIMP.
You don’t technically need G’MIC to make this work, but the process of manually creating a Vivid Light layer is tedious and error-prone in GIMP right now.
Colors → Invert So right now, we should be looking at this on our canvas: Now that we’ve got our inverted “Temp” layer, we just need to apply a little blur.