Painting in the depth map can be quite challenging especially when you have depth changes occuring in two directions. A good example of this is the forehead which recedes horizontally as well as vertically. That can be challenging to scribble correctly. Here, I chose to use the gradient option in the pencil toolbox. The difficulty though is getting the length fade of the gradient correct. The alternative is to make a selection and run the gradient fill, which should work also.
Here, I am gonna go the extra mile and smooth the depth map with Edge Preserving Smoothing 2 (EPS2).
3D scene rendered in Gimpel3D. As you can see, it's far from being perfect but that should do for our purposes.
Note that Gimpel3D seems to have trouble loading depth maps that are indexed (256 levels of gray), so it may be a good idea to convert the depth map to RGB prior to loading it into Gimpel3D.