The documentation for Gimpel3D is not the best but you can pretty much figure out what all the tools do thanks to the html "help" inside the program. There are some things that are not very well explained though, in particular, the "Alignment Options" and "Gap Fill".
Something that should be stressed: Whatever you do in Gimpel3D in terms of modeling, when you look straight on at the scene from the virtual camera (the red dot when Views->Show Camera is checked), you will always see the 2d picture you loaded as if the scene were flat. This explains why things may look weird when you look at the 3d scene from different view points.
In the following youtube video, I've tried to reconstruct the Mona Lisa (actually, a close-up) using Gimpel3D:
Here's the corresponding depth map:
As you can see, there are some problems: the nose is a bit on the manly side and the eye sockets are set way too deep. There's also a depth discontinuity between the face and the hair on both sides. It's not really Gimpel3D's fault as the model file for the human head is probably not the greatest for our Mona Lisa.
Here's another youtube video where I attempt to model the nose of the Mona Lisa using 5 planes (layers) and the "orientation" tool:
Here's the depth map:
I guess I could have also used the "scale" tool but kinda forgot about it when I made the video. In theory, one could model the whole face manually using a bunch of facial planes (see books about drawing the human head). That would certainly be a bit tedious, especially knowing that there's the capability to project onto models. If you don't have a model file handy for an object in the foreground you want to render precisely, I am afraid that there might not be much of a choice. One could also probably use the "contour extrusion" tool and the "anchor points" tool to speed up the "dimensionalization" of an object. I've tried those and they work quite well (as described), but obviously these tools have their own limitations.