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You're getting the banding because you're rendering to a compressed and limited color-space format. Try adding a little bit of noise to the layer with the gradient.
First, make sure that you're rendering to ProRez 422 HQ at a minimum. Make sure quality is all the way up.
Second, make sure that you're not introducing artifacts with your key by rendering a few frames of just the gradient.
Third, give your project the best option for success by working in 32bit and make sure that all effects are 32 bit compatible.
If all else fails, add a bit of noise. You don't need much, just enough to make the encoder work on the pixels a bit harder.
I swirched to 32 bit and tried going to Pro Res HQ, but that didn't quite fix it. ProsRes 4444 (don't they mean 444?) did the trick. HUGE file sizes though!
32 bit improves video quality no end, partticlarly the keying, though I've had to go over the whole project and tweak the key settings, and in 32-bit mode that's taken a bit of time...
I'm going to be appying a vignette, a zoom, and some other stuff, maybe a 3D HUD, so I could have added some noise now, but I think this footage might be useful for something else, so I wanted to keep a clean copy.