borderWidth property

Specifies the border width of tail.

Defaults to 0.

Widget build(BuildContext context) {
   return Container(
       child: SfRadialGauge(
         axes:<RadialAxis>[RadialAxis
         ( pointers: <GaugePointer>[
            NeedlePointer( tailStyle:
                TailStyle(borderWidth: 2,width: 10,length: 0.2,
                borderColor: Colors.red)
          )])]
       ));
}