Hi Derrick,
In Gantt, there is a limitation while adding a field/column at runtime using column menu with data from the JSON file, it is not possible to retain the updated data or the fields which were added at runtime, since on a page load the data from the JSON file will be redefined and the data saved at runtime will be lost. Therefore we recommend you to define the data in the SQL database to retain the updated data.
Please let us know, if you require any other assistance on this.
Regards,
Jayakumar D