Hello,
The dev team added a way to specify the page orientation when you export the data. To use that feature, you need to add the additional_settings parameter. You can manually specify the sizes in the width and height parameters or use the format and landscape parameters: https://docs.dhtmlx.com/gantt/api__gantt_exporttopdf.html
Here is an example: http://snippet.dhtmlx.com/5/fb46146a0
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