This tutorial explains using Smudge tool to paint angle-like wings in GIMP. No filters (like Gaussian blur) are required for postprocessing, albeit you can try adjust the result.

Here is the sequence of steps.

1. Create a new image of any size (for example, 580x580 pixels). You may fill it with black colour using “Advanced Options → Fill with...” in “Create new image” dialogue, or fill it later using Bucket Fill tool (Shift+B).

2. Create a new transparent layer (Layer → New layer...) and name it “Wing”.

3. Draw a white line similar to the picture below in the layer “Wing”. Use Paintbrush tool (P) with the following parameters: set colour to white, set size between 5×5 and 11×11 – as you wish; or use Ink tool (K) with the following parameters: set colour to white, size – to 2 or a little larger, set angle to 0, type and shape – round. Your drawing must look like the upper outline of the wing.

4. Now select the Smudge tool (S), set the same size as for Paintbrush tool. You can experiment with its “Rate” parameter, but it’s better to set it to 50 +-10. Smudge your “Wing” layer as it’s shown on the picture below:

Continue to smudge until you see something like on the picture below. For the final touches you can decrease the size of the tool and increase its rate.

5. And here is it. Certainly, it doesn’t look like real wing, but it looks nice I think.

6. If you wish you can add a few touches to your wing to make it look real. Create another transparent layer and draw a black line in it as you see on the picture below:

Select Smudge tool (S) again. But now you should smudge your black line top-down as it’s shown on the picture. You should begin to smudge above the line:

7. You can repeat #6 near the side of the wing, but decrease the size of Paintbrush tool (P) and Smudge tool (S). As the result you’ll have picture similar to the example below:

8. Now you can copy your wing and edit its shape with Perspective tool (Shift+P). And then feel free to go ahead and attach it to some body.