Dependencies are defined based on Issue Links. Changing dependencies creates or removes links between issues (link type is defined in the Gantt configuration).
Gantt supports Finish to Start, Start to Start, Finish to Finish and Start to Finish dependencies. To create a Finish to Start dependency, mouse over the right end of the task bar and when the dot appears, click it and drag the mouse to the task you want to link it to. Once the mouse is released, the dependency (and the corresponding Jira link) is created.
To create other types of dependencies, use the Link button in the toolbar. Click it to show the dialog, select the parent and child issues, the link type and click Create Dependency (issue currently in focus is set as parent by default).
Clicking the link arrow on the chart will show the Dependency Details panel.
To delete the dependency, click the Remove link in the Dependency Details panel.