If we take advantage of the equation for a circle:

r = sqrt(x * x + y * y)

We can get circular gradients.

For x we use 2U - 1.
For y we use 2V - 1.

The Sqrt node shows the circular gradient. We then plug that into Blender or ColorRamp. Again, we can use Bias to accelerate or retard the gradient in r.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)