In the Marketplace description, under the 'Pricing overview' section, it says that add-on is not available for OnDemand - https://marketplace.atlassian.com/plugins/de.polscheit.jira.plugins.gantt
It is not valid for OnDemand.
You might want a quick look at https://marketplace.atlassian.com/plugins/app/jira/highest-rated?hosting=ondemandand tinker with the filters to find stuff, but I don't think there's any Gantt options for Jira in there.
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