I had this problem as well and found the above reply from Teodor Taushanov absolutely useless.  He clearly hadn't read the user's email.

On another forum I found a much more helpful reply from Infragistics David Negley in which he solves the problem with the sample code below.    I attach here for anyone else that wishes to have a custom tooltip associated with their gantt TimeEntry.

using System;
using System.Collections;
using System.Data;
using System.Web;
using System.Web.UI;
using Infragistics.UltraChart.Data;
using Infragistics.UltraChart.Shared.Styles;
using Infragistics.UltraChart.Resources;
using Infragistics.UltraChart.Resources.Appearance;
public partial class _Default : Page, IRenderLabel
{
    protected void Page_Load(object sender, EventArgs e)
    {
        this.UltraChart1.ChartType = ChartType.GanttChart;
        this.UltraChart1.Tooltips.FormatString = "<MY_LABEL>";
        Hashtable labelHash = new Hashtable();
        labelHash.Add("MY_LABEL", this);
        this.UltraChart1.LabelHash = labelHash;

        GanttSeries series1 = new GanttSeries("Series 1");
        GanttItem item1 = series1.Items.Add("Item 1");
        GanttTimeEntry timeEntry1 =
item1.Times[item1.Times.Add(DateTime.Now, DateTime.Now.AddDays(1))];
        this.UltraChart1.Series.Add(series1);
    }
    string IRenderLabel.ToString(Hashtable context)
    {
        DateTime startTime = (DateTime)context["START_TIME"];
        DateTime endTime = (DateTime)context["END_TIME"];

        int seriesIndex = (int)context["DATA_ROW"];
        int itemIndex = (int)context["DATA_COLUMN"];
        int timeEntryIndex = (int)context["TIME_ENTRY_INDEX"];

        GanttSeries series = this.UltraChart1.Series[seriesIndex] as
GanttSeries;
        GanttItem item = series.Items[itemIndex];
        GanttTimeEntry timeEntry = item.Times[timeEntryIndex];

        return series.ToString() + "//" + item.ToString() + "//" +
timeEntry.ToString();
    }
}