Unsharp Mask will increase contrast, but it works by exaggerating local differences. You probably do want to use Unsharp Mask in the processing of an image like this to increase the amount of contouring, but it's not the best way to go about fixing the major tonal issue.

You can't really say that there isn't enough total contrast in the image, since you have blown-out white areas and some absolute blacks. The problem is that there isn't enough contrast in the "interesting" parts of the picture — the hair isn't dark enough, and the part of the white shirt that isn't a highlight isn't quite light enough. You can use a curve to fix that (I don't have Gimp, and I figure that Gimp users are pretty tired of seeing Photoshop examples everywhere, so I'm using the Curves adjustment from Paint Shop Pro 9 just to show that it doesn't have to be Photoshop to work):

Note the "S" shape; that increases the amount of contrast in the mid-range of tones, making things that are not quite black darker and things that are not quite white lighter. That will make the picture "pop" quite a bit more than it does with the original tones:

The curves adjustment can go quite a bit further than I've gone here without damaging the image in any way. Honestly, the adjustment looks a lot bigger with the before and after in the same place (turning the layer with curves on and off) than it does here, so I probably should have been more extreme with the example. I was leaving room for the next step...

If the shapes aren't quite "3D" enough at that point, you can use a large-radius Unsharp Mask at a fairly low strength to boost the contours a bit, but whether you want to (I probably would) and how much/what settings is a matter of taste and personal preference. But you'll be starting with an image that has a better contrast range where you want it, so you shouldn't wind up with something that looks over-processed.