In this article I will explain what I did, in order to create a glow map for the ghoul textures. For this purpose I used GIMP.

1. After opening the ghoul body's diffuse map in GIMP, I used the Brush Tool and painted in white all the parts that I don't want to be glowing(like underwear).
2. Colors -> Invert. All dents and damages on the skin are darker on the texture. With this option they will become lighther and will later receive a glow. Parts painted in white(like the underwear from the previous step) become black and will have no glow.
3. Colors -> Desaturate -> Color To Gray. Enable option "Enhance Shadows". Makes some details on the texture stand out, but also makes the whole image noisy.
4. Filters -> Blur -> Gaussian Blur. This will reduce the "noise" from the effects of the previous filter. Set Size X and Y to 1.00 or less, because it will look too much blurred.
5. Colors -> Brightness & Contrast. Reduce brightness to -127, increase contrast, but not to the max. You decide how much exactly, depending on the details you want visible.
6. Colors -> Hue & Saturation. Reduce Lightness to -100, because the texture is still too bright.
7. File -> Export As. Same name as the diffuse map, just change "d" to "g". Example: GhoulFemaleBody_g.DDS. Use BC1/DXT1 compression method and enable generating of mip maps. Don't close the program yet, first test how the glow map looks in the game.
8. After testing, if there are still small parts of the texture that you don't want to have glow, you can paint them in black with the Paintbrush Tool. Export and test again. And use the Undo History to go back a few steps if necessary, or you want to do something differently.

Note: Other types of blur can be used instead of Gaussian, like Median, or Filters -> Enhance -> Noise Reduction, or a combination. And it makes difference if the blur effect is applied before or after the brightness/contrast adjustment. You can experiment with this, it may require going into the game multiple times to check how it looks. For the Classic Ghouls glow map I used Noise Reduction before Brightness/Contrast, and after - light Gaussian Blur(Size X/Y set to 0.50 or 0.45).

I decided it's better to not colorize the texture in green, but leave it in black and white. The white parts are the ones, which will emit the glow. This way, strength and color of the glow can be customized easily through the BGSM material file, without editing and compressing the texture.

Important settings in the BGSM material:
1. Glow Texture - set the path to the glow map.
2. Emittance Enabled - turn on glowing.
3. Emittance Color - change the color of the glow effect. If the glow texture is in color, then choose white.
4. Emittance Multiplier - change the strength of the glow.
5. Glowmap - enable in order to use a glow texture. With a glow map you can control, which part of the texture(and therefore the model) will emit a glow.
6. Skin Tint - disable it, otherwise the glow will not work. The result from turning it off is - changing skin tone will have no effect, extras/tints will not work on the face anymore.