Hello Jinhua,
Unfortunately, there is no built-in way to do that. You need to implement a custom solution to fit the timeline and the lightbox to the phone screen.
Or you can add a custom lightbox and define your styles for that:
https://docs.dhtmlx.com/gantt/desktop__custom_edit_form.html
Here are examples of how it might be implemented:
http://snippet.dhtmlx.com/76e1d7197
http://snippet.dhtmlx.com/f5b4014eb
https://snippet.dhtmlx.com/a1b5ebf55