Kill me if it's not made with a vector editor (like inkscape)
![Razz :P](./images/smilies/icon_razz.gif)
Usually when you encounter something with very pure lines, perfect curves, no grain, ultrasmooth gradients and quite minimalistic and/or illustrative then it is vector art, in fact I'm listing here the major pros (which are also cons, in a way) of this genre.
It could be made with gimp yes, your tools will be: paths, gradients and a careful observation of what illustrative and/or minimalistic style implies. But I suspect it will be more feasible in inkscape.