Hi Tiago,
the Jaspersoft Studio for pixel perfect reports can be found in the Palette under "Components Pro"-"Wigets Pro".
These widget are provided by the third-party FusionCharts library.
Unfortunately, there is no documentation available how the Jaspersoft wizard translates the configurations into what is understood by FusionCharts.
Moreover, in the version shipped with the Jaspersoft server uses Flash to render the gantt, which imo makes it useless as this technology is not reccomended. Also, exporting the Flash, e.g. to PDF will not work.
Please be aware that if you tell the server to render it in JavaScript, the Gantt will crash the browser.
In short:
You will have to build the Gantt with some other technology and integrate it to your report, e.g. via dashboard.
Regards
Georg