This is a demo tutorial. While there is no step-by-step commentary available (yet), the live demo below is fully functional. Feel free to open it for full source code.
Normally, Pie Chart supports radial gradients only. This demo shows a clever trick with events that modify linear gradient rotation, enabling use of this type of fill on slices.
See the Pen amCharts 4: Donut with radial gradient by amCharts team (@amcharts) on CodePen.24419