I can see that you prefer a semi-realistic style.
I like her smile and the look in her eyes.
The expression on her face is warm and friendly, i also liked that!
You did really good with her nose, the shape and shades are just working out great!
The shape of the eyes has a touch of your own unique style you should really flesh that out.
But now to the more important part of a critique:
Her skin looks a bit pale/cold i would have tried more warm, red tones.
It is difficult to find the right skin colour and it differs from person to person, but you should spend some time to find it.
Pay attention to the position of her eyes.
It looks like the right eye is falling out of her head.
it is a difficult perspective where you have to decide whether you make the eye shorter, so that it fits in the head, or make it more round to give a 3 dimensional feel (which works in the fewest cases)
Her eyebrows look a bit weird, like they were shaved and start growing again.
They have too less volume, but maybe its her style i dont know...
What bothers me the most is her hair.
The shading is fine, but the hair itself looks stiff and undynamic like it is glued or something.
Give the hair more flow, volume, substance.
The strands at her neck are really thick at their tips and lets them look smeary.
altogether you did good with this one, but there are some issues you should work on to be really successfull with this style of art.
I would recoment you to use some other program except GIMP
"Easy paint tool SAI" is a powerfull and relatively cheap alternative if you have paypal.
Firstly, I'd like to thank you for the extensive critique and in-depth critique.
I completely agree with you about the skin, I just couldn't manage to get a better tone out of it even though I've tried several shades. I might try looking at some references the next time in order to get a better result.
To tell the truth, I'm sort of new when it comes to hair in general, so her eyebrows and, well, hair are the reflection of that. Coming from a more manga-esque style I still have ways to go to get the desired effects on this field..
About the program, I believe GIMP will do just fine, plus being a Linux user all the major alternatives are unavailable to me. I have, however, worked with SAI before and am currently in the process of getting its pressure sensitivity to work under Linux. I will probably work with it more should I succeed.
Und noch einmal, vielen Dank für Ihre Zeit