To start with, you've gotten the shape of the face and the hair looking really good and realistic as well as the highlights on the glasses lenses as well as the shading on the clothes cos I know clothes can be awkward from my experiences!
However, to improve, I'd probably add a little more shading around the eyes and nose as well as probably studying eyes just a little bit because (from my personal experience) I found it helps. Another factor, is the eyes, there is a lot more to them than you've shown, HOWEVER, the iris's,pupils and eye highlights are looking good!!
Another thing you may want to consider is the placement of eyebrows...Obviously this depends on whether you had a picture reference, though if you didn't, I'd say than the brow bone needs to be lightened a bit more. And if you didn't have a picture I'd shape the eye brows just a bit more.
The best way to practice getting the different parts of the face the right tone is just to practice with a picture reference and just use the airbrush on GIMP. That's how my recent pictures have been done, but their just doodles really But if you look at a picture you will see not all the face is the same colour/shade.
overall though its really good seeing as you're just trying out with GIMP (out of curiosity which version is it? I use 2.6!) Much better than what I started off with!! And is a very nice piece of art!! I hope this helps and isn't too constructive I'm not that good at writing critiques!!
(also, can I just say that the above crit. Depend on whether you were aiming for realism or not, but either way its a great piece of art, and I have to say that if this is one of your first attempts with GIMP then you've done extremely well!!)