Ok, as you don't comment on my comments, I'll leave a version of official answer.
Now, Gant diagram is not a part of JavaFX 2+, and I don't know about an existing samples, where it is already built. So, the answer is - to build it by yourself.
Some part of JavaFX is open source, javafx controls, and javafx-charts projects are open-sourced as well. They can be found here : http://openjdk.java.net/projects/openjfx/
You can observe code of bar charts, line charts (and, may be, a scroll pane - for large charts), and it should become clear enough to understand, how to build a Gant chart yourself. It will require some coding, yes..
Possibly, if it will be created well, you can even go through the process and contribute it into open jfx.