Here is a sum-up of the steps to be taken to successfully migrate Gantt Suite to Jira Cloud. Prior to migration, please study Atlassian documentation to make sure you understand the whole process.
1. Launch Jira Cloud instance.
2. Install Cloud version of Gantt Suite for Jira on the launched instance.
2.1. Navigate to Apps → Gantt to verify that the app is installed.
3. Log in as admin to your Jira Server/Data-Center.
3.1. Update Gantt Suite for Jira to the latest Server/Data-Center version.