If I understand you correctly, you want to customize the lightbox, which appears on
creating/editing the task, am I right?
You can add some additional fields to the lightbox, using the
gantt.config.lightbox.sections, for example, in a snippet below I added the “Priority” field to the task, and can change it using the lightbox:
You can change the styles of the default lightbox elements using CSS, here is an example(HTML tab):
Also, you can create the fully custom lightbox, here is the link to the docs,
lightbox to add/remove sections and controls:
Templates of the Lightbox to specify the format of lightbox elements: