Hi
Mukarram,
Thanks
for contacting Syncfusion Support.
To
modify any column, you can use the column template and render the tooltip
component inside it to show the tooltip when hovering the icon. You can change
the tooltip content using ‘content’ property in the tooltip component.
You
can refer to our user guide on column template for more information: https://ej2.syncfusion.com/angular/documentation/gantt/columns/column-template
For
more information on our tooltip component, you can refer to this link: https://ej2.syncfusion.com/angular/documentation/tooltip/getting-started#initialize-tooltip-within-a-container
We
have prepared a sample using Angular to demonstrate how to achieve this. You
can refer to it by visiting this link
https://stackblitz.com/edit/angular-syaast-a8dvy9?file=index.html,app.component.html,app.component.ts,main.ts,app%2Fapp.module.ts
Here
is a code snippet to help you get started

Regards,
Divya