Hi Tony , I do not know of any filter in Gimp that would do that from a photo. I think that this image is drawn or painted or least heavily worked after using some filters. It is very stylised. Look at the flow of the pant's creases. The other thing is that not all color gradations are with a black outline. The inking is most probably done by hand. A photo can be used to get some sort of image outlines with filters and then requires some painting but I can not get much better than this. Perhaps Inkcape is a better way to go but I know very little about it. Perhaps some of the IS persons can have a go at this. Where does your image come from?
Just reading Rod's post. Yes I remember a tutorial in Gimptalk about vexel art (author Pixilated ? or something like that, Sorry Pix) Here is an attempt to show a starting point.