I'm sorry to say that it isn't currently possible to install plugins into the OnDemand environment unless they have been bundled by our developers, as per our [OnDemand plugin policy|http://confluence.atlassian.com/display/AOD/Atlassian+OnDemand+Plugin+Policy], and unfortunately there aren't any Gantt Chart plguins bundled in the OnDemand environment. We do have open feature requests to ask that both the Gantt Chart plugins are bundled in a future release of OnDemand and therefore I would suggest that you watch/vote/comment on those requests to encourage our developers to prioritise their implementation:
All the best,
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