I checked several versions: 1.1.2, 1.5, 1.6.4, and 1.7. Not one provides a collapsible Gantt chart. I don't believe they could, because they all use a simple HTML file that references .jpg files containing the rendition of the Gantt chart. You would have to construct .jpg files showing the Gantt chart in every possible configuration, which becomes impractical for a project of appreciable complexity. There's also no reference to such a feature in the manual or the release notes. You can export just the outline as an OmniOutliner file, and the result will be collapsible (in OmniOutliner, or exported as dynamic HTML from OmniOutliner) but it will not have a Gantt chart.