Use a proper tool like Photoshop (or even GIMP) and gradients become something you can do in seconds!
Making the gradient itself was relatively simple, thanks to an online tool. The trouble was making the panels and other non-black lines match the gradient.
At any rate, I redid the drawing and actually got it better than the first time.
This one is for my present-day AU, a Sukhoi Superjet 100 operated by the Wikipedian low-cost carrier Wiqid.