All Gantt chart data is stored in an attachment of the page that the Gantt chart is on.
When you face the following situation, you want to restore the Gantt chart from the attachment.
You removed the macro by mistake.
You want to restore an older version of the Gantt chart
How to restore Gantt chart
Between Step 2 and 4 isn’t needed to restore an older version.
Download the attachment that keeps all data of the Gantt chart
Open the attachment view of the page
Download the json file. (If you want to restore an older version, you download the target version.)
Create a new empty Gantt chart on the page.
Open the attachment view and get the attachment name of the Gantt chart you created at step 2.
Rename the file you downloaded at step 1 into the attachment name you got at step 3.
Upload the attachment to the page.