flutter_svg/example/assets/simple/radial_gradient_focal.svg

<svg viewBox="0 0 150 150"  xmlns="http://www.w3.org/2000/svg">
  <defs>
    <radialGradient id="radial"
      cx="50%" cy="50%" fx="50%" fy="15%" r="50%">
      <stop offset="0%" style="stop-color: red;"/>
      <stop offset="50%" style="stop-color: green;"/>
      <stop offset="100%" style="stop-color: blue;"/>
    </radialGradient>
   
    </defs>
    <rect x="10" y="10" width="120" height="120"
      style="fill: url(#radial); stroke: black;"/>
</svg>