There are different ways in which you can add tasks using the Gantt module. You can add tasks one by one or add a group of tasks to the scope of the Box. When you use the Gantt's interface, besides adding Jira issues, you can also add Basic tasks which will only be displayed using the App.
Task duration
If you do not specify the task duration while creating a new task, it will be set to 1 day.
By default, the App requires two points on the timeline (in other words two dates) to generate a taskbar and determine the start and endpoints. Those points in time are the task's "Start Date" and "End Date" and are stored as "Start Date" and "End Date" custom fields by default.
If a task with no start and/or end dates is added to the scope of a Box, the App will pick the "Original Estimate" value to calculate the duration and set the task's "Start Date" to the "Creation Date".
Following this logic, if no "Original Estimate" value is available, the App will again use the "Creation Date" as "Start Date", and when the "End Date" is empty, then it picks the resolution date (if the issue was resolved) instead. This usually means that a task will be set as a 1-day duration starting on the "Creation Date". With the "Original Estimate" mapped as the start or end dates, it will represent the duration of a task.
Box scheduling settings and scheduling mode of tasks can automatically overwrite start/end dates.
Creating new tasks
Jira tasks
Create a new Jira task (issue) using the task creation screen. Any issue type can be added as a task. If a task is already selected, the new task will be added just beneath it on the same level in the hierarchy.
Depending on the project synchronization settings, adding new tasks might update different fields mapped as start and end dates. To learn more go to the Fields configuration page.
For example: If the "Start Date" field is not synchronized or it is not added to the appropriate issues screen, when you add a task there will not be any update of the "Start Date" field. However, when the "Start Date" is synchronized, adding a task without setting the start date estimate will result in a task update, and the "Creation Date" will be used as the "Start Date" (the "Creation Date" will also be used as the "End Date" if the task is not resolved).
In the cloud version of the App, you cannot create a Jira sub-task on Gantt - this functionality works only for the Server version of the App.
More information on the differences between cloud and server versions of the App can be found here.
Make sure you have selected the correct Jira project. Otherwise, you might unintentionally create a task in the wrong place.
If the App overwrites the manually specified start/end dates, make sure to verify Box scheduling settings and scheduling mode of tasks.
For example: if the parent task is in the "auto top-down" mode, the task period will be automatically altered during creation.
A new Jira task is created.
Start/End dates are set to 2023.
The task period has been automatically adjusted.
Basic tasks
Create a basic task and add it to the list of tasks. Such tasks will not be synchronized with your Jira or connected tools. For example, you will not see a basic task in your Jira project.
Task templates
This requires you to create a template first using the Task Template Creation menu in the Box configuration.
Import from file
Import your task templates from other tools like MS Project, MS Excel, or a CSV file. For detailed instructions on how to import tasks from a file, refer here.
Clone existing scope
Use your existing Boxes as templates and clone the scope to a different Box. As data needs to be stored in Jira projects, make sure that your project settings are the same as the source project.
Inline task adding
You can add tasks directly using the task list, and they will appear on the selected level in the hierarchy.
Only Jira issues can be added using the inline task adding.
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Include existing tasks from a task source
You can add existing tasks to a Box by adjusting its /wiki/spaces/BIGGANTT/pages/315885158 Items that already exist in tools such as Jira and Trello can be included in your Box.
Go to: Add task > Manage scope definition to make adjustments. 
Empty box
When a box doesn't contain tasks you see the following splash screen:
Create manually
This option lets you create new tasks.
Click on the tile:
Basic task
To create a basic task select it from the drop-down:
Jira issue
To create a new task in a Jira project select it from the drop-down:
Add existing tasks from Jira
This option lets you add existing tasks from a Jira project.
Click on the tile:
Scope definition settings appear in a pop-up:
To access advanced settings click the Advanced options link at the bottom: