Hello Chinmaya,
If a task property is changed, you need to repaint it to see the changes. There is no way to repaint only part of the row. Gantt will repaint the whole row.
If you want to show a different value in the grid column, you need to use the template function of the column configuration. It allows showing any text or HTML elements in the cell instead of the actual value: https://docs.dhtmlx.com/gantt/desktop__specifying_columns.html#datamappingandtemplates