If you want to only remove project tasks from a Gantt but keep the task in the project as such, do as follows:
The task will only be removed from the Gantt chart.
The task will still be in the Kanban board/task list.
To delete a task completely from both the Gantt chart and the project as such, do the following:
WARNING This deletes the task immediately without InLoox asking you to confirm the deletion!
Now, the task is removed from the Gantt chart and deleted from the project.
WARNING Both, the removal and the deletion of tasks from the Gantt chart cannot be undone!
Deleting an activity with linked tasks will also remove the tasks from the Gantt chart and - if the check mark is set - will also delete those tasks from the project. More details in the help article on deleting project planning elements.