You need to use
RowDataBound of GridView for this.
protected void GridView_RowDataBound(Object sender, GridViewRowEventArgs e)
{
DataControlRowType rtype = e.Row.RowType;
if (rtype == DataControlRowType.DataRow && rtype != DataControlRowType.Footer
&& rtype != DataControlRowType.Separator && rtype != DataControlRowType.Header
&& rtype != DataControlRowType.Pager)
{
e.Row.ToolTip = "This text needs to shown on mouseover of the row!";
}
}