Gantt Cloud Add-on can be accessed through JIRA Projects menu, via Gantt Charts link. This link is available for every user with "Browse projects" permissions. Once loaded, you should see Add-on main page similar to this one:

Top menu with filters section - you can cotnrol what is rendered on the chart using various filters. Read more about filtering issues
Configuration button - opens cofiguration dialog where you can change some Add-on's settings or archive old projects
Active filters panel - list of filters that are enabled and active. More about using filters
Header toggle buttons - allows to switch between weeks and months or week days and day of month displayed in chart's top header

The state of toggle buttons and selected filters will be remembered and restored next time you activate the Add-on.
Left header - with all issues grouped by projects.

Although it is possible to have on the screen gantt charts for more that one project, there are some limitations however. If you have dependencies between issues from different projects they won't be rendered.
Main chart - you can scroll the chart area horizontally. Date range is calculated automatically to cover all rendered issues. Click on the issue to see more details. By dragging it, you can change the start date. Read more about setting start dates

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