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Level adjust green, red, blue individually. Then HSV decompose to extract V (value) channel.
In more detail (for GIMP 2.*):
1. Layer -> Colours -> Levels
Now tweak max. input and gamma for red, blue and green channels. I put max in to the highest value in the image and gamma to between 2 and 1.5, depending on visual appearance.
2. Filters -> Colours -> Decompose. Choose HSV and uncheck "Decompose to layers" (i.e. make three separate images not 3 layers in one image). Hue and saturation are basically just noise, so chuck them out. The remaining value channel is what I've posted.
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