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.
This demo shows how we can use multiple series to simulate multi-category bars on a Gantt chart.
See the Pen amCharts 4: Gantt chart with columns that span multiple categories by amCharts team (@amcharts) on CodePen.24419