You can apply "Selective Gaussian Blur" with big enough radius and treshold=255. You will get something like this:
Making the radius bigger wipes off more details.
Unfortunately GIMP has no idea, do you want some black lines to be saved. You must redraw them or make a selection which covers only the areas to be blurred or blur it in pieces.
Here the middle blade was selected and blurred (selective gaussian blur) separately. The selection was inverted to blur the rest. Blur radius was 20 for the blade and 40 for the rest.
Doing it well takes a lot of work. Blurring wipes off all fine surface texture. The result will be "non glossy plastic" at best, if you do not add anything as a substitute. As well you can go back to Blender, as already suggested or even more preferably make a 3D model which isn't a polygon mesh, but a smooth NURBS surface, like this: