Well the easiest way to do that is code like this:
First of all you have to calculate center coords as your requirements are. (This is often the center of the node). Then you calculate the length of your current coord.
float r = length(cc_FragTexCoord1 - centerCoord);
gl_FragColor = mix(startColor, endColor, r);
What this code does is mixing start color and end color with and uses radial length as an alpha so in a result you will have a full-radius gradient of the node.
But the result that it will provide to you might not what you want since gradation will be visible and also there is no calculation for radius, but I hope you get the idea.