Brighten dark shadow in a photo

Duplicate the lowest original layer of the image. Change the Mode of this duplicated layer to Overlay, for using this layer as a retouching Contrast Mask.

Use the Layers » Color » Desaturate tool to remove the saturation of colors in the retouching Contrast Mask. Use the Layers » Color » Invert tool for inverting the brightness in the desaturated, colorless Contrast Mask.

Blur the desaturated and inverted Contrast Mask with the Filters » Blur » Gaussian Blur -filter. I used a radius of 15x15 px for blurring the "Contrast Mask Layer" in my image.

Use the Layer » Mask » Add Layer Mask -dialog, for adding a semi transparent Layer Mask to the shadow correcting Contrast Mask to make the effect of retouch more diffuse, soft and photo(no) natural. Use the setting Grayscale copy of layer as the layer mask type.

Tip: You can duplicate the layer with the Contrast Mask to brighten the shadow in the image even more.

Duplicate the lowest original layer of the image. Move the duplicated layer to the top of all layers and change the Mode of this moved layer to Hue -mode, for keeping the hue of colors natural to the original image.

Duplicate the upper, hue saving layer and change the mode of this copy to the Saturation -mode, for keeping the saturation of colors natural to the original image.

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xoxo

aL