Setting progress or description is not supposed to show anything. There's a different way to add a tooltip however. Check out StepElement#addTooltip(String). And to get StepElement, see Gantt#getStepElement(String uid).
For progress, with StepElement, you could maybe customize it with the Vaadin's Element API to show a progress bar. Showing progress is not supported out of the box.
Please report bugs/features/questions in https://github.com/tltv/gantt-flow/issues, if you find more. Thanks.