1. Colors > Split Channel > Split to RGB. Keep the R channel. (An alternative is to make a blend of the red and green channels with Colors > Channel Mixer.)

2. Clarify at a setting of 3 to 5.  Do this two times.

3. Histogram Adjustment: Set Low and High clip limits to no more that 0.01%. Set gamma so that detail in the shadows is just barely visible but not so high that the image is lightened overall too much. (Try a value of 1.2.)

4. Colors > Increase Color Depth > 16 Million Colors.

5. Colors > Colorize, hue 160 and saturation 15-20.