flutter_svg/example/assets/simple/radial_gradient.svg

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox="0 0 800 400" version="1.1"
  xmlns="http://www.w3.org/2000/svg">
  <desc>Example radgrad01 - fill a rectangle by referencing a
           radial gradient paint server</desc>
  <g>
    <defs>
      <radialGradient id="MyGradient" gradientUnits="userSpaceOnUse" cx="400" cy="200" r="300" fx="400" fy="200">
        <stop offset="0%" stop-color="red" />
        <stop offset="50%" stop-color="blue" />
        <stop offset="100%" stop-color="red" />
      </radialGradient>
    </defs>

    <!-- Outline the drawing area in blue -->
    <rect fill="none" stroke="blue" x="1" y="1" width="798" height="398"/>

    <!-- The rectangle is filled using a radial gradient paint server -->
    <rect fill="url(#MyGradient)" stroke="black" stroke-width="5" x="100" y="100" width="600" height="200"/>
  </g>
</svg>