...
Having logged in as JIRA system administrator and having downloaded, installed the Gantt-Chart add-on and get a license key via the Atlassian Marketplace, you have to configure it once: please click on the configure-button within the expanded Gantt-Chart section of the "universal plugin manager" (UPM) inside JIRA administration, modify the settings according to your local needs or take over the defaults and in both cases, click on the apply/finish button at the bottom (without clicking on apply, nothing will be changed within the local JIRA nor any default settings are stored/taken over):
Administrative steps to be done manually:
...