Here's how I do it:
1. Load your ship chassis in GIMP (the base model)
2. Copy the image into a new file (select all -> paste as -> new image. It shouldn't be a problem to select the whole image, as the surroundings should be transparent.)
3. Save the new image as a shield file (might as well start now, eh?) and create a new (transparent) layer atop the layer with the ship image. It should be pretty much exactly the same size.
4. Cover the new transparent layer using the ellipse-select tool. The tool should consist of a rectangle with an ellipse inside. Drag the corners of the rectangle to the very edges of the layer, and this should shape the ellipse accordingly.
5. Create a new gradient using the gradients dialogue. (If you want, you can extract one of the original shield images from the resource.dat file, load it in gimp, and use the color picker tool). The left side of the gradient should be completely transparent (made possible by using the alpha slider) and the other side should be the color you want for the shield, with maybe a small amount of alpha.**
6. Hopefully you still have the ellipse selection (it should remain as long as you don't make another selection). Click on the "Blend" tool and load your gradient. Change the pattern style to "shaped (spherical)."
7. Drag the blend tool over the selected area. Hopefully, the effect will resemble the shields in-game (color on the outside, fading to transparent on the inside). You might need to reblend it once or twice (for me, 20,000 times lol) to get a desirable configuration. If it appears inside out, you need to flip the gradient accordingly, but it should work. If it fills the whole image instead of the ellipse, you'll probably need to reselect.
You can use the ship image we uploaded before to see how the shield fits on the ship beforehand. If its too small, you need to make the dimensions of the shield layer bigger, and vice versa.
Long tutorial but hope this helps.