Unless I have put values in, dont change from default
Load Photo into Gimp
Colours - Levels - auto input levels - while in there change the value top slider (move the centre arrow to the right so clamp is .70 (Darkens it a little and brings out more detail)
Duplicate your layer and desaturate it in luminance mode.
Go to Filters, enhance, highpass. 155.7 std dev and 1.962 contrast
Now put it in overlay mode and drop the opacity while in layers to 46. (Sharpens)
Merge your two layers.
Now using colours, shadows-highlights take the shadows up to 52.85 and the highlights down to -52.33.
Filters enhance - sharpen unsharp mask
You could also try a little burn
and convolve 
on the faces with a small brush. Burn to bring out the features more and convolve to sharpen them even more.
Load Photo into Gimp
Colours - Levels - auto input levels - while in there change the value top slider (move the centre arrow to the right so clamp is .70 (Darkens it a little and brings out more detail)
Duplicate your layer and desaturate it in luminance mode.
Go to Filters, enhance, highpass. 155.7 std dev and 1.962 contrast
Now put it in overlay mode and drop the opacity while in layers to 46. (Sharpens)
Merge your two layers.
Now using colours, shadows-highlights take the shadows up to 52.85 and the highlights down to -52.33.
Filters enhance - sharpen unsharp mask
You could also try a little burn
and convolve on the faces with a small brush. Burn to bring out the features more and convolve to sharpen them even more.
