A method using the Gimp to do Contrast Masking like the descriptions of using Photoshop here and here.	What ?
	Duplicate Layer Create a duplicate Layer ( Ctrl-c on the Layer Form) Rename it and move it to the top of the layer stack.
	Image : Colors : Desaturate Image : Colors : Invert
	Change Layer Mode to Overlay
	Now apply a blur Filter : Blur : Gaussian Blur . Try a Blur radius of 10 to start
	Without "Contrast Mask" Layer
	With "Contrast Mask" Layer