Hello,
Where exactly do you need to add the ellipsis property?
For example the
text-overflow: ellipsis is a default behaviour of point names displayed on yAxis (on the left) in Gantt chart, but not in tooltip.
Please create a simple demo with your issue. Without the code we won't be able to help you. You can pick any of the default Gantt demos, recreate the issue and post it as a reply.
Also, use the forum and a Google search since the ellipsis in Highcharts is a very popular topic, and most likely you will find the answer to your question.
Forum search: search.php?keywords=ellipsis
Gantt Demos: https://www.highcharts.com/demo/gantt
Regards